Migrating to 4.0

Note

This describes the upcoming 4.0 release. Most of the renames below can be adopted from 3.9 onward, well before 4.0 lands — the new spellings and the old ones both work during the deprecation window, so you can migrate incrementally and silence the warnings as you go.

diff-diff 4.0 removes the deprecated surface that 3.9 shipped warnings for, merges three pairs of estimators, and flips two families of inference defaults. Every change is tracked as a row in docs/v4-deprecations.yaml, and the appendix below is checked against that ledger by tests/test_v4_matrix.py — if a row is added, moved, or rescheduled and this page is not updated, CI fails.

What changes

Area	What changes	Rows	Where
aggregate-postfit	Aggregation moves off fit() onto post-fit results.aggregate(...)	13	§4
alias-table	Six export aliases are removed in favour of their canonical class names	6	§7
constructor-hygiene	covariates= moves from the constructor to fit()	1	§7b
df-convention-flip	The df_convention default flips to "cluster" on seven estimators - numbers move	7	§6
diagnostic-family	Bacon is re-homed out of the estimator roster into the diagnostics family	1	§7b
field-flip	Nine results containers rename overall_att to the canonical att	9	§5
function-wrappers	Eight module-level wrapper functions are removed; call the classes	8	§7
merge-ddd	StaggeredTripleDifference is absorbed by TripleDifference	5	§2
merge-mpd	MultiPeriodDiD is absorbed by TwoWayFixedEffects(event_study=True)	5	§2
merge-qdid	QDiD is absorbed by ChangesInChanges(method="qdid")	2	§2
obligation-sdid-params	Two SyntheticDiD constructor params (inert since 3.0.0) are removed	3	§7b
obligation-warning-retirements	A transition FutureWarning stops being emitted	1	§7b
policy-auto-cluster	TwoWayFixedEffects auto-clusters on unit unless you pass cluster=	1	§6
renames-cohort	cohort= becomes first_treat=	1	§3
renames-col-suffix	The *_col suffix is dropped across 27 parameters and one field	27	§3
renames-control-group	clean_control= becomes control_group=	2	§3
renames-dcdh	dCDH’s group=/controls= become unit=/covariates=	4	§3
renames-level	Aggregation-level params and their accepted values unify on level=/"event_study"	4	§3
renames-post	The post-dummy time= becomes post=	3	§3
renames-robust-drop	robust= is dropped in favour of vcov_type= - read §3, the translation is per-estimator	4	§3
results-contract	A legacy sentinel field is retired in favour of the unified event-study surface	1	§5

The three merges

Each merge keeps one class and retires the other. The retired class still works in 3.9 and warns.

MultiPeriodDiD → TwoWayFixedEffects(event_study=True)

# 3.x
from diff_diff import MultiPeriodDiD
results = MultiPeriodDiD().fit(data, outcome="y", unit="id", time="period", treatment="treat")

# 4.0
from diff_diff import TwoWayFixedEffects
results = TwoWayFixedEffects().fit(
    data, outcome="y", unit="id", time="period", treatment="treat",
    event_study=True, spec="pooled", post_periods=[3, 4, 5],
)

Two things are easy to get wrong here, and both change numbers:

• spec="pooled" reproduces the MultiPeriodDiD design. The new default is spec="within", which adds unit fixed effects. That moves point estimates as well as standard errors on unbalanced panels or with covariates — the two specs coincide only in the restricted equivalence case (balanced panel, no covariates, simultaneous adoption). “Only SEs move” is not the migration message.

• post_periods= is required in event-study mode. MultiPeriodDiD defaulted to a midpoint split of the calendar, which is a silent guess; the merged mode makes you state the treatment boundary. spec="pooled" is also the only spec valid for repeated cross-sections.

StaggeredTripleDifference → TripleDifference

# 3.x
from diff_diff import StaggeredTripleDifference
results = StaggeredTripleDifference().fit(data, outcome="y", unit="id", time="period",
                                          first_treat="g", eligibility="p")

# 4.0
from diff_diff import TripleDifference
results = TripleDifference().fit(data, outcome="y", unit="id", time="period",
                                 first_treat="g", partition="p")

The staggered fit parameters are keyword-only on the merged class. The results container is the unified TripleDifference shape; the 2x2x2 design reads as a degenerate single-ATT view of it.

QDiD → ChangesInChanges(method=”qdid”)

# 3.x
from diff_diff import QDiD
results = QDiD(n_bootstrap=200, seed=42).fit(data, outcome="y", treatment="treat", time="post")

# 4.0
from diff_diff import ChangesInChanges
results = ChangesInChanges(method="qdid", n_bootstrap=200, seed=42).fit(
    data, outcome="y", treatment="treat", time="post")

Only the class spelling is deprecated, not the estimator. method="qdid" is a fully supported comparison mode and emits no warning of its own — the numbers are unchanged, because it is the same engine. method="cic" is the default and encodes Athey–Imbens’ recommendation.

Renamed parameters

Most renames are mechanical: the new spelling already exists, so you can adopt it today and the old one keeps working until 4.0. See the appendix for the full list, and the R Comparison page for how these names map onto the R packages.

Three families need more than a search-and-replace:

robust= → vcov_type= is per-estimator, not a single rule. Verified against the current release:

estimator	robust=True	robust=False
DifferenceInDifferences, LinearRegression	already the hc1 default — drop it	vcov_type="classical"
TripleDifference	drop it	drop it — only hc1 is accepted, so robust= never changed inference, and vcov_type="classical" raises
HeterogeneousAdoptionDiD	vcov_type="hc1"	drop it — non-robust is its legacy default

time= → post= carries a semantic change, and the old name survives. On DifferenceInDifferences, TripleDifference and TwoWayFixedEffects, time= used to mean a 0/1 post dummy. The name is not removed — it is repurposed to mean the calendar column. From 4.0, passing time= in static mode (TwoWayFixedEffects) or 2x2x2 mode (TripleDifference) raises ValueError rather than being silently reinterpreted. If you pass a post dummy, rename it to post=; if you pass a calendar column to an event-study or staggered fit, leave it alone.

Dropped parameters are not always deletions. A parameter with no successor in the appendix may still need a replacement call — see its Fix cell.

Post-fit aggregation

Aggregation moves off fit() onto the results object:

# 3.x
results = CallawaySantAnna().fit(data, ..., aggregate="event_study")

# 4.0
results = CallawaySantAnna().fit(data, ...)
event_study = results.aggregate("event_study")

DiagnosticReport / BusinessReport need no migration step of their own: where applicable, their event-study-gated checks derive the surface internally via the result’s post-fit aggregate('event_study') when the raw event_study_effects field is absent, so moving a fit off the fit-time keyword no longer silently disables those checks. The routing is estimator-specific: CallawaySantAnna derives for parallel trends, pre-trends power, and sensitivity; ImputationDiD and TwoStageDiD for parallel trends (pretrends=True fits) and heterogeneity; ContinuousDiD for heterogeneity. StackedDiD and SunAbraham always populate the raw surface (nothing to derive), and ChaisemartinDHaultfoeuille’s pre-period checks read placebo_event_study directly.

Warning

Bootstrapped fits: CallawaySantAnna, DMLDiD, and EfficientDiD are covered; the remaining recompute adopters are not yet. On CallawaySantAnna, DMLDiD (which inherits the CS replay channel), and EfficientDiD, the post-fit recompute levels now REPLAY the fit-time multiplier bootstrap from the fit-retained RNG state (percentile inference matching a fit-time aggregation to floating-point reassociation) — no fit-time keyword needed; only pre-replay legacy pickles and artifacts moved across the Rust/NumPy weight backend fail closed with a refit message. On ImputationDiD, TwoStageDiD and ContinuousDiD, the recompute levels still raise NotImplementedError when the fit used n_bootstrap > 0 — keep the fit-time call there for now and track their open TODO.md rows. aggregate("simple") — and, on its five adopters, aggregate("total") (3.10; DMLDiD joined in 3.11 — panel non-survey fits only: RCS fits, declared-survey_design= fits, and bare-cluster= fits with incomplete treated support fail total closed) — does relay, and StackedDiD, ChaisemartinDHaultfoeuille and HeterogeneousAdoptionDiD are unaffected — their aggregate() is a pure view over stored fields.

Results fields

Nine containers rename overall_att to the canonical att. The new accessor already works today, so this migration can be done immediately; the old name becomes a warning-emitting property at 4.0 and is removed at 5.0.

att = results.att          # canonical, works now
att = results.overall_att  # warns from 4.0, removed at 5.0

The whole inference quintet moves, not just the point estimate. Wherever a container carried overall_* inference fields alongside overall_att, they flip to the canonical names in the same step:

old	new
overall_att	att
overall_se	se
overall_t_stat	t_stat
overall_p_value	p_value
overall_conf_int	conf_int

Note

ContinuousDiDResults is the outlier: its sibling fields are spelled overall_att_se, overall_att_t_stat, overall_att_p_value and overall_att_conf_int — with the att infix — and they flip to the same canonical names. If you grep for overall_se you will miss them.

Inference defaults that move numbers

Two changes alter results without changing any call:

• df_convention flips to "cluster" on seven estimators. Pass df_convention="residual" to reproduce 3.x numbers exactly.

• Panel estimators auto-cluster at unit when cluster= is omitted. Pass cluster=False to disable it — None and omission both mean “auto-cluster”, so cluster=None does not reproduce unclustered 3.x standard errors. Cross-sectional 2x2 estimators stay HC-robust unless you pass cluster=. Note the TWFE event-study mode has auto-clustered since 3.9 (it shipped that way with the merge), so this flip changes the static and other panel paths, not that one.

Removed functions and aliases

Eight module-level wrapper functions and six export aliases are removed. Both were thin indirections: call the estimator class directly, or import the canonical name. The appendix lists each one with its target.

Remaining 4.0 changes

Smaller items that do not fit the families above — two inert SyntheticDiD constructor parameters, the covariates= constructor-to-fit() move, a retired transition warning, the Bacon roster re-homing, and the family-wide anticipation validation below. The appendix lists the ledger-derived removals/flips; [M-144], [M-145], and [M-146] are behavior tightenings, and [M-147] is an additive policy. These have no removal/deprecation fields and appear here only.

• WooldridgeDiD(unsupported_period_action="error") ([M-147]) refuses periods lacking comparison support before removing them. The default "drop" preserves filtering and warnings, so existing callers need no migration. The policy applies to all three methods independently of rank_deficient_action; other identification checks still apply. Survey validation runs first, then "error" raises ValueError for unsupported periods; "drop" retains the survey-domain NotImplementedError when removal would be required.

• anticipation is validated across the family ([M-144], landing at 4.0): whole-valued floats that previously fit identically to their integer now raise — pass the int; bool and negative/non-integer values raise too. Accepted numpy integers are normalized, so the public anticipation attribute and get_params()["anticipation"] are now always built-in int (previously a numpy scalar survived). WooldridgeDiD’s error message text changed to the shared wording, and its constructor now reports a bad bootstrap_weights/vcov_type/df_convention before a bad anticipation (the ordering flipped).

• pscore_trim is validated via a shared helper ([M-145], landing at 4.0): ContinuousDiD tightens from [0, 0.5) to (0, 0.5) — pscore_trim=0 (which disabled the overlap clip keeping IPW/DR weights finite) now raises, its message wording changed, and non-real-scalar inputs (None, strings, Decimal/Fraction, 1-element arrays) raise ValueError instead of TypeError (or being silently accepted, for the 1-element array). CallawaySantAnna gains the same type guard; TripleDifference, CallawaySantAnna, and ContinuousDiD now store the value coerced to built-in float; LWDiD’s message wording changed. The deprecated StaggeredTripleDifference keeps its permissive construction shape.

• summary(alpha=...) / print_summary(alpha=...) on the results classes family-wide ([M-146], landing at 4.0; the staggered family first, then the non-staggered summaries - DiDResults incl. SpilloverDiDResults, MultiPeriodDiDResults, SyntheticDiDResults, TripleDifferenceResults, TROPResults, ContinuousDiDResults): a value different from the fit-time alpha now raises ValueError instead of silently relabeling the confidence-interval header over fit-time stored intervals; alpha=0.0, previously swallowed by a falsy-or default, raises too. Re-fit at the desired alpha instead. SyntheticControlResults.summary joins the contract as a carve-out: its alpha was a dead no-op (it prints no alpha-based interval; the displayed confidence set is keyed on its own stored gamma), and a non-fit value now raises like the rest of the family.

One pending decision: the DIFF_DIFF_SOLVE_OLS_FASTPATH environment default has a go/no-go due at 4.0 that has not been made. If it lands on, it is a numerics change and will be documented then; “evaluated, kept off” is an equally valid outcome, so it carries no appendix row today.

Codemod

The mechanical keyword renames can be applied with a regex sweep. This table covers only identifier renames — it deliberately excludes dropped parameters (no successor to write), results-field renames (.groups → .units would rewrite every pandas groupby(...).groups in your code), and accepted-value renames, which change strings rather than names.

find	replace
\boutcome_col\s*=	outcome=
\bdose_col\s*=	dose=
\btime_col\s*=	time=
\bunit_col\s*=	unit=
\bfirst_treat_col\s*=	first_treat=
\brunning_col\s*=	running=
\btreatment_col\s*=	takeup=
\bweight_col\s*=	weights=
\bcohort\s*=	first_treat=
\bclean_control\s*=	control_group=
\bcontrols\s*=	covariates=
\baggregation\s*=	level=

Note treatment_col maps to takeup, not treatment — it is the one *_col parameter whose replacement is not just the suffix dropped.

Three renames are deliberately NOT in the table, because a global replace would corrupt working code. Each is a rename only in a specific call:

rename	where it applies	why it cannot be global
group= → unit=	ChaisemartinDHaultfoeuille.fit, twowayfeweights	TripleDifference.fit takes a group= that is not renamed
time= → post=	DifferenceInDifferences.fit, permutation_test, leave_one_out_test	time= survives everywhere else as the calendar column — see the renamed-parameters section
aggregate= → post-fit call	the aggregation family	not a rename at all; it moves off fit()

Review every hit even in the safe table: these are word-boundary matches on common words.

Already shipped in 3.9

These landed in 3.9 and need no 4.0 action, but they moved behaviour and are easy to mistake for 4.0 changes:

Numbers moved. The ETWFE reference-period fixes (unidentified-cohort exclusion, and a fail-closed guard on designs with no estimable post-treatment cell), the clustered-CR1 K_reference convergence, and the tail-df consolidation.

Fail-closed validation. n_bootstrap semantics unified; inference="wild_bootstrap" without cluster= now raises; TripleDifference(pscore_trim=0) is rejected; an all-NaN Wooldridge overall ATT raises rather than returning NaN. These are refusals, not default changes — no numeric default moved.

Additive API and container changes. The unified EventStudyResults surface, the AggregationResult container, and the Diagnostic marker base on the diagnostic result roster. The alias-diet __getattr__ shim also landed here: CDiD, Stacked and Gardner still import but now emit a FutureWarning, and they no longer appear in dir()/vars().

Appendix: every 4.0 change

One row per ledger row that requires action at 4.0 — 108 in total. Old and New are the ledger’s own locators. An em dash in New means the ledger names no successor locator; it does not mean no action is required, so read the Fix cell.

Row	Group	Old	New	Fix
M-020	aggregate-postfit	diff_diff:CallawaySantAnna.fit[aggregate]	diff_diff:CallawaySantAnnaResults.aggregate	Move aggregate= off fit() onto post-fit results.aggregate(...). Bootstrapped fits replay the fit-time bootstrap post-fit - see the aggregation section.
M-021	aggregate-postfit	diff_diff:ImputationDiD.fit[aggregate]	diff_diff:ImputationDiDResults.aggregate	Move aggregate= off fit() onto post-fit results.aggregate(...). On a bootstrapped fit the recompute levels raise today - see the aggregation section.
M-022	aggregate-postfit	diff_diff:TwoStageDiD.fit[aggregate]	diff_diff:TwoStageDiDResults.aggregate	Move aggregate= off fit() onto post-fit results.aggregate(...). On a bootstrapped fit the recompute levels raise today - see the aggregation section.
M-023	aggregate-postfit	diff_diff:EfficientDiD.fit[aggregate]	diff_diff:EfficientDiDResults.aggregate	Move aggregate= off fit() onto post-fit results.aggregate(...). Bootstrapped fits replay the fit-time bootstrap post-fit - see the aggregation section.
M-024	aggregate-postfit	diff_diff:StackedDiD.fit[aggregate]	diff_diff:StackedDiDResults.aggregate	Move aggregate= off fit() onto post-fit results.aggregate(...).
M-025	aggregate-postfit	diff_diff:ContinuousDiD.fit[aggregate]	diff_diff:ContinuousDiDResults.aggregate	Move aggregate= off fit() onto post-fit results.aggregate(...). On a bootstrapped fit the recompute levels raise today - see the aggregation section.
M-026	aggregate-postfit	diff_diff:ChaisemartinDHaultfoeuille.fit[aggregate]	diff_diff:ChaisemartinDHaultfoeuilleResults.aggregate	Move aggregate= off fit() onto post-fit results.aggregate(...).
M-027	aggregate-postfit	diff_diff:HeterogeneousAdoptionDiD.fit[aggregate]	diff_diff:HeterogeneousAdoptionDiDResults.aggregate	Move aggregate= off fit() onto post-fit results.aggregate(...).
M-117	aggregate-postfit	diff_diff:CallawaySantAnna.fit[balance_e]	diff_diff:CallawaySantAnnaResults.aggregate[balance_e]	Move balance_e= off fit() onto post-fit results.aggregate(...). Bootstrapped fits replay the fit-time bootstrap post-fit - see the aggregation section.
M-118	aggregate-postfit	diff_diff:ImputationDiD.fit[balance_e]	diff_diff:ImputationDiDResults.aggregate[balance_e]	Move balance_e= off fit() onto post-fit results.aggregate(...). On a bootstrapped fit the recompute levels raise today - see the aggregation section.
M-119	aggregate-postfit	diff_diff:TwoStageDiD.fit[balance_e]	diff_diff:TwoStageDiDResults.aggregate[balance_e]	Move balance_e= off fit() onto post-fit results.aggregate(...). On a bootstrapped fit the recompute levels raise today - see the aggregation section.
M-120	aggregate-postfit	diff_diff:EfficientDiD.fit[balance_e]	diff_diff:EfficientDiDResults.aggregate[balance_e]	Move balance_e= off fit() onto post-fit results.aggregate(...). Bootstrapped fits replay the fit-time bootstrap post-fit - see the aggregation section.
M-139	aggregate-postfit	diff_diff:did_had_pretest_workflow[aggregate]	—	Remove aggregate=; the battery is inferred from panel shape - two periods select the overall battery, more than two select the event-study battery. It is not selected post-fit (HADPretestReport.aggregate is a metadata field, not a method).
M-060	alias-table	EventStudy	—	Import TwoWayFixedEffects(...).fit(..., event_study=True) - the alias is dropped, not retargeted instead of the EventStudy alias.
M-061	alias-table	QDiDResults	—	Import ChangesInChangesResults instead of the QDiDResults alias.
M-064	alias-table	SDDD	—	Import TripleDifference (staggered mode) instead of the SDDD alias.
M-132	alias-table	CDiD	—	Import ContinuousDiD instead of the CDiD alias.
M-133	alias-table	Stacked	—	Import StackedDiD instead of the Stacked alias.
M-134	alias-table	Gardner	—	Import TwoStageDiD instead of the Gardner alias.
M-084	constructor-hygiene	diff_diff:ContinuousDiD[covariates]	diff_diff:ContinuousDiD.fit[covariates]	covariates= moves from the constructor to fit() - pass it at fit time.
M-004	df-convention-flip	diff_diff:DifferenceInDifferences[df_convention]	—	The df_convention default flips to "cluster"; pass df_convention="residual" to keep 3.x numbers.
M-005	df-convention-flip	diff_diff:TwoWayFixedEffects[df_convention]	—	The df_convention default flips to "cluster"; pass df_convention="residual" to keep 3.x numbers.
M-006	df-convention-flip	diff_diff:LinearRegression[df_convention]	—	The df_convention default flips to "cluster"; pass df_convention="residual" to keep 3.x numbers.
M-128	df-convention-flip	diff_diff:SunAbraham[df_convention]	—	The df_convention default flips to "cluster"; pass df_convention="residual" to keep 3.x numbers.
M-129	df-convention-flip	diff_diff:WooldridgeDiD[df_convention]	—	The df_convention default flips to "cluster"; pass df_convention="residual" to keep 3.x numbers.
M-130	df-convention-flip	diff_diff:StackedDiD[df_convention]	—	The df_convention default flips to "cluster"; pass df_convention="residual" to keep 3.x numbers.
M-131	df-convention-flip	diff_diff:ImputationDiD[df_convention]	—	The df_convention default flips to "cluster"; pass df_convention="residual" to keep 3.x numbers.
M-090	diagnostic-family	diff_diff:BaconDecomposition	—	Bacon moves out of the estimator roster into the diagnostics family - update docs references and imports of the roster, not call sites.
M-050	field-flip	diff_diff:CallawaySantAnnaResults.overall_att	diff_diff:CallawaySantAnnaResults.att	Read att instead of overall_att; the canonical accessor already works today.
M-051	field-flip	diff_diff:SunAbrahamResults.overall_att	diff_diff:SunAbrahamResults.att	Read att instead of overall_att; the canonical accessor already works today.
M-052	field-flip	diff_diff:ImputationDiDResults.overall_att	diff_diff:ImputationDiDResults.att	Read att instead of overall_att; the canonical accessor already works today.
M-053	field-flip	diff_diff:TwoStageDiDResults.overall_att	diff_diff:TwoStageDiDResults.att	Read att instead of overall_att; the canonical accessor already works today.
M-054	field-flip	diff_diff:StackedDiDResults.overall_att	diff_diff:StackedDiDResults.att	Read att instead of overall_att; the canonical accessor already works today.
M-055	field-flip	diff_diff:EfficientDiDResults.overall_att	diff_diff:EfficientDiDResults.att	Read att instead of overall_att; the canonical accessor already works today.
M-056	field-flip	diff_diff:WooldridgeDiDResults.overall_att	diff_diff:WooldridgeDiDResults.att	Read att instead of overall_att; the canonical accessor already works today.
M-057	field-flip	diff_diff:ChaisemartinDHaultfoeuilleResults.overall_att	diff_diff:ChaisemartinDHaultfoeuilleResults.att	Read att instead of overall_att; the canonical accessor already works today.
M-058	field-flip	diff_diff:ContinuousDiDResults.overall_att	diff_diff:ContinuousDiDResults.att	Read att instead of overall_att; the canonical accessor already works today.
M-070	function-wrappers	diff_diff:imputation_did	—	Call the estimator class directly instead of imputation_did.
M-071	function-wrappers	diff_diff:two_stage_did	—	Call the estimator class directly instead of two_stage_did.
M-072	function-wrappers	diff_diff:stacked_did	—	Call the estimator class directly instead of stacked_did.
M-073	function-wrappers	diff_diff:trop	—	Call the estimator class directly instead of trop.
M-074	function-wrappers	diff_diff:synthetic_control	—	Call the estimator class directly instead of synthetic_control.
M-075	function-wrappers	diff_diff:triple_difference	—	Call the estimator class directly instead of triple_difference.
M-076	function-wrappers	diff_diff:bacon_decompose	—	Call the estimator class directly instead of bacon_decompose.
M-077	function-wrappers	diff_diff:chaisemartin_dhaultfoeuille	—	Call the estimator class directly instead of chaisemartin_dhaultfoeuille.
M-013	merge-ddd	diff_diff:StaggeredTripleDifference	diff_diff:TripleDifference.fit[first_treat]	See the merges section for the worked before/after.
M-014	merge-ddd	diff_diff:StaggeredTripleDiffResults	—	Read the unified TripleDifference results shape (a degenerate single-ATT view for 2x2x2) instead of this container.
M-085	merge-ddd	diff_diff:TripleDifference.fit[time]	—	In 2x2x2 mode, time= raises - pass post= for the 0/1 post dummy. time= now means the staggered calendar column only.
M-140	merge-ddd	diff_diff:TripleDifference.fit[aggregate]	diff_diff:TripleDifferenceResults.aggregate (not yet implemented)	No successor yet - post-fit aggregate() is not implemented on the DDD container (open TODO.md row). Keep the fit-time call until it lands.
M-141	merge-ddd	diff_diff:TripleDifference.fit[balance_e]	diff_diff:TripleDifferenceResults.aggregate[balance_e] (not yet implemented)	No successor yet - post-fit aggregate() is not implemented on the DDD container (open TODO.md row). Keep the fit-time call until it lands.
M-010	merge-mpd	diff_diff:MultiPeriodDiD	diff_diff:TwoWayFixedEffects.fit[event_study]	See the merges section for the worked before/after.
M-011	merge-mpd	diff_diff:MultiPeriodDiDResults	—	Read the unified event-study surface on the merged TwoWayFixedEffects results instead of this container.
M-012	merge-mpd	diff_diff:PeriodEffect	—	Superseded by the unified event-study representation - read the event-study surface rather than per-period PeriodEffect objects.
M-016	merge-mpd	diff_diff:MultiPeriodDiDResults.period_effects	—	Not removed at 4.0: it becomes a FutureWarning property view over the unified event-study surface on the successor container, and is removed at 5.0. Migrate the read to the successor class.
M-083	merge-mpd	diff_diff:TwoWayFixedEffects.fit[time]	—	In static mode (event_study=False), time= raises - pass post= for the 0/1 post dummy. time= now means the calendar column only.
M-015	merge-qdid	diff_diff:QDiD	diff_diff:ChangesInChanges[method]	See the merges section for the worked before/after.
M-143	merge-qdid	diff_diff:ChangesInChangesResults.estimator	diff_diff:ChangesInChangesResults.method	See the merges section for the worked before/after.
M-001	obligation-sdid-params	diff_diff:SyntheticDiD[lambda_reg]	—	Drop it. It has been IGNORED since 3.0.0, so 3.x already auto-computes regularization - do NOT copy its value into zeta_omega=, which activates an override that was inert and changes weights, ATT and inference. Set zeta_omega= only as a deliberate new choice.
M-002	obligation-sdid-params	diff_diff:SyntheticDiD[zeta]	—	Drop it. It has been IGNORED since 3.0.0, so 3.x already auto-computes regularization - do NOT copy its value into zeta_lambda=, which activates an override that was inert and changes weights, ATT and inference. Set zeta_lambda= only as a deliberate new choice.
M-003	obligation-sdid-params	diff_diff:SyntheticDiDResults.placebo_effects	diff_diff:SyntheticDiDResults.variance_effects	Read variance_effects instead of placebo_effects.
M-007	obligation-warning-retirements	diff_diff.estimators	—	The MultiPeriodDiD e=-1 transition FutureWarning stops being emitted; drop any warning filter that suppressed it.
M-080	policy-auto-cluster	diff_diff:TwoWayFixedEffects[cluster]	—	Panel estimators auto-cluster at unit when cluster= is omitted; pass cluster=False to disable it and keep unclustered 3.x standard errors (None/omission means auto-cluster, not off).
M-032	renames-cohort	diff_diff:WooldridgeDiD.fit[cohort]	diff_diff:WooldridgeDiD.fit[first_treat]	Rename the keyword: cohort= becomes first_treat=.
M-035	renames-col-suffix	diff_diff:HeterogeneousAdoptionDiD.fit[outcome_col]	diff_diff:HeterogeneousAdoptionDiD.fit[outcome]	Rename the keyword: outcome_col= becomes outcome=.
M-036	renames-col-suffix	diff_diff:HeterogeneousAdoptionDiD.fit[dose_col]	diff_diff:HeterogeneousAdoptionDiD.fit[dose]	Rename the keyword: dose_col= becomes dose=.
M-037	renames-col-suffix	diff_diff:HeterogeneousAdoptionDiD.fit[time_col]	diff_diff:HeterogeneousAdoptionDiD.fit[time]	Rename the keyword: time_col= becomes time=.
M-038	renames-col-suffix	diff_diff:HeterogeneousAdoptionDiD.fit[unit_col]	diff_diff:HeterogeneousAdoptionDiD.fit[unit]	Rename the keyword: unit_col= becomes unit=.
M-039	renames-col-suffix	diff_diff:HeterogeneousAdoptionDiD.fit[first_treat_col]	diff_diff:HeterogeneousAdoptionDiD.fit[first_treat]	Rename the keyword: first_treat_col= becomes first_treat=.
M-040	renames-col-suffix	diff_diff:RegressionDiscontinuity.fit[outcome_col]	diff_diff:RegressionDiscontinuity.fit[outcome]	Rename the keyword: outcome_col= becomes outcome=.
M-041	renames-col-suffix	diff_diff:RegressionDiscontinuity.fit[running_col]	diff_diff:RegressionDiscontinuity.fit[running]	Rename the keyword: running_col= becomes running=.
M-042	renames-col-suffix	diff_diff:RegressionDiscontinuity.fit[treatment_col]	diff_diff:RegressionDiscontinuity.fit[takeup]	Rename the keyword: treatment_col= becomes takeup=.
M-088	renames-col-suffix	diff_diff:RDPlot.fit[outcome_col]	diff_diff:RDPlot.fit[outcome]	Rename the keyword: outcome_col= becomes outcome=.
M-089	renames-col-suffix	diff_diff:RDPlot.fit[running_col]	diff_diff:RDPlot.fit[running]	Rename the keyword: running_col= becomes running=.
M-094	renames-col-suffix	diff_diff:RegressionDiscontinuityResults.treatment_col	diff_diff:RegressionDiscontinuityResults.takeup	Read takeup instead of treatment_col on the results object.
M-098	renames-col-suffix	diff_diff:joint_pretrends_test[outcome_col]	diff_diff:joint_pretrends_test[outcome]	Rename the keyword: outcome_col= becomes outcome=.
M-099	renames-col-suffix	diff_diff:joint_pretrends_test[dose_col]	diff_diff:joint_pretrends_test[dose]	Rename the keyword: dose_col= becomes dose=.
M-100	renames-col-suffix	diff_diff:joint_pretrends_test[time_col]	diff_diff:joint_pretrends_test[time]	Rename the keyword: time_col= becomes time=.
M-101	renames-col-suffix	diff_diff:joint_pretrends_test[unit_col]	diff_diff:joint_pretrends_test[unit]	Rename the keyword: unit_col= becomes unit=.
M-102	renames-col-suffix	diff_diff:joint_pretrends_test[first_treat_col]	diff_diff:joint_pretrends_test[first_treat]	Rename the keyword: first_treat_col= becomes first_treat=.
M-103	renames-col-suffix	diff_diff:joint_homogeneity_test[outcome_col]	diff_diff:joint_homogeneity_test[outcome]	Rename the keyword: outcome_col= becomes outcome=.
M-104	renames-col-suffix	diff_diff:joint_homogeneity_test[dose_col]	diff_diff:joint_homogeneity_test[dose]	Rename the keyword: dose_col= becomes dose=.
M-105	renames-col-suffix	diff_diff:joint_homogeneity_test[time_col]	diff_diff:joint_homogeneity_test[time]	Rename the keyword: time_col= becomes time=.
M-106	renames-col-suffix	diff_diff:joint_homogeneity_test[unit_col]	diff_diff:joint_homogeneity_test[unit]	Rename the keyword: unit_col= becomes unit=.
M-107	renames-col-suffix	diff_diff:joint_homogeneity_test[first_treat_col]	diff_diff:joint_homogeneity_test[first_treat]	Rename the keyword: first_treat_col= becomes first_treat=.
M-108	renames-col-suffix	diff_diff:did_had_pretest_workflow[outcome_col]	diff_diff:did_had_pretest_workflow[outcome]	Rename the keyword: outcome_col= becomes outcome=.
M-109	renames-col-suffix	diff_diff:did_had_pretest_workflow[dose_col]	diff_diff:did_had_pretest_workflow[dose]	Rename the keyword: dose_col= becomes dose=.
M-110	renames-col-suffix	diff_diff:did_had_pretest_workflow[time_col]	diff_diff:did_had_pretest_workflow[time]	Rename the keyword: time_col= becomes time=.
M-111	renames-col-suffix	diff_diff:did_had_pretest_workflow[unit_col]	diff_diff:did_had_pretest_workflow[unit]	Rename the keyword: unit_col= becomes unit=.
M-112	renames-col-suffix	diff_diff:did_had_pretest_workflow[first_treat_col]	diff_diff:did_had_pretest_workflow[first_treat]	Rename the keyword: first_treat_col= becomes first_treat=.
M-113	renames-col-suffix	diff_diff:trim_weights[weight_col]	diff_diff:trim_weights[weights]	Rename the keyword: weight_col= becomes weights=.
M-043	renames-control-group	diff_diff:StackedDiD[clean_control]	diff_diff:StackedDiD[control_group]	Rename the keyword: clean_control= becomes control_group=.
M-095	renames-control-group	diff_diff:StackedDiDResults.clean_control	diff_diff:StackedDiDResults.control_group	Read control_group instead of clean_control on the results object.
M-033	renames-dcdh	diff_diff:ChaisemartinDHaultfoeuille.fit[group]	diff_diff:ChaisemartinDHaultfoeuille.fit[unit]	Rename the keyword: group= becomes unit=.
M-034	renames-dcdh	diff_diff:ChaisemartinDHaultfoeuille.fit[controls]	diff_diff:ChaisemartinDHaultfoeuille.fit[covariates]	Rename the keyword: controls= becomes covariates=.
M-097	renames-dcdh	diff_diff:twowayfeweights[group]	diff_diff:twowayfeweights[unit]	Rename the keyword: group= becomes unit=.
M-114	renames-dcdh	diff_diff:ChaisemartinDHaultfoeuilleResults.groups	diff_diff:ChaisemartinDHaultfoeuilleResults.units	Read units instead of groups on the results object.
M-044	renames-level	diff_diff:WooldridgeDiDResults.to_dataframe[aggregation]	diff_diff:WooldridgeDiDResults.to_dataframe[level]	Rename the keyword: aggregation= becomes level=.
M-086	renames-level	diff_diff:WooldridgeDiDResults.aggregate[type]=event	diff_diff:WooldridgeDiDResults.aggregate[type]=event_study	Change the accepted value: "event" becomes "event_study".
M-087	renames-level	diff_diff:WooldridgeDiDResults.summary[aggregation]	—	summary() unifies to the library-wide summary(alpha=None) signature - drop aggregation= and select the level on aggregate() instead.
M-136	renames-level	diff_diff:LPDiDResults.to_dataframe[level]=event	diff_diff:LPDiDResults.to_dataframe[level]=event_study	Change the accepted value: "event" becomes "event_study".
M-030	renames-post	diff_diff:DifferenceInDifferences.fit[time]	diff_diff:DifferenceInDifferences.fit[post]	Rename the keyword: time= becomes post=.
M-137	renames-post	diff_diff:permutation_test[time]	diff_diff:permutation_test[post]	Rename the keyword: time= becomes post=.
M-138	renames-post	diff_diff:leave_one_out_test[time]	diff_diff:leave_one_out_test[post]	Rename the keyword: time= becomes post=.
M-045	renames-robust-drop	diff_diff:DifferenceInDifferences[robust]	—	Drop robust=True (already the hc1 default); robust=False becomes vcov_type="classical".
M-046	renames-robust-drop	diff_diff:TripleDifference[robust]	—	Drop it. TripleDifference accepts only vcov_type="hc1", so robust= never changed its inference - do NOT write vcov_type="classical", which raises.
M-047	renames-robust-drop	diff_diff:HeterogeneousAdoptionDiD[robust]	—	HAD’s legacy default is non-robust: robust=True becomes vcov_type="hc1"; drop robust=False.
M-115	renames-robust-drop	diff_diff:LinearRegression[robust]	—	Drop robust=True (already the hc1 default); robust=False becomes vcov_type="classical".
M-093	results-contract	diff_diff:CallawaySantAnnaResults.event_study_effects	diff_diff:EventStudyResults	The sentinel is retired - read the unified EventStudyResults surface instead of the raw field.