diff --git a/analysis/matlab/run_all.m b/analysis/matlab/run_all.m
index 9cd6d11..88f5873 100644
--- a/analysis/matlab/run_all.m
+++ b/analysis/matlab/run_all.m
@@ -7,7 +7,8 @@ scenarios = {'unmerged_full', 'unmerged_d0_10', ...
              'mergeB2_full', 'mergeB2_d0_10', ...
              'lme_unmerged_full', 'lme_unmerged_d0_10', ...
              'lme_mergeA2_full', 'lme_mergeA2_d0_10', ...
-             'lme_mergeB2_full', 'lme_mergeB2_d0_10'};
+             'lme_mergeB2_full', 'lme_mergeB2_d0_10', ...
+             'lme_unmerged_d0_13', 'lme_mergeA2_d0_13', 'lme_mergeB2_d0_13'};
 
 for i = 1:numel(scenarios)
     fprintf('\n\n### Running scenario: %s ###\n', scenarios{i});
diff --git a/analysis/matlab/tdcs_glm.m b/analysis/matlab/tdcs_glm.m
index 762366d..65e0ca6 100644
--- a/analysis/matlab/tdcs_glm.m
+++ b/analysis/matlab/tdcs_glm.m
@@ -10,9 +10,11 @@ function tdcs_glm(scenario)
 %     mergeA2_full,  mergeA2_d0_10,
 %     mergeB2_full,  mergeB2_d0_10
 %   or a linear "days x tDCS" mixed model (Box-B2 vs Box-A2, per the paper):
-%     lme_unmerged_full, lme_unmerged_d0_10,
-%     lme_mergeA2_full,  lme_mergeA2_d0_10,
-%     lme_mergeB2_full,  lme_mergeB2_d0_10
+%     lme_unmerged_full, lme_unmerged_d0_10, lme_unmerged_d0_13,
+%     lme_mergeA2_full,  lme_mergeA2_d0_10,  lme_mergeA2_d0_13,
+%     lme_mergeB2_full,  lme_mergeB2_d0_10,  lme_mergeB2_d0_13
+%   The _d0_13 window ends where Box-A2 runs out of data, giving both anchor
+%   groups equal day coverage (the fair window for the interaction).
 %
 %   Each case below is a self-contained, independently invocable call of
 %   the same three-function pipeline; they are enumerated explicitly
@@ -83,6 +85,18 @@ switch scenario
         L = tdcs_lme(tdcs_scenario_data('mergeB2_d0_10'), cfg);
         tdcs_lme_report(L, 'lme_mergeB2_d0_10', cfg);
 
+    case 'lme_unmerged_d0_13'
+        L = tdcs_lme(tdcs_scenario_data('unmerged_d0_13'), cfg);
+        tdcs_lme_report(L, 'lme_unmerged_d0_13', cfg);
+
+    case 'lme_mergeA2_d0_13'
+        L = tdcs_lme(tdcs_scenario_data('mergeA2_d0_13'), cfg);
+        tdcs_lme_report(L, 'lme_mergeA2_d0_13', cfg);
+
+    case 'lme_mergeB2_d0_13'
+        L = tdcs_lme(tdcs_scenario_data('mergeB2_d0_13'), cfg);
+        tdcs_lme_report(L, 'lme_mergeB2_d0_13', cfg);
+
     otherwise
         error('tdcs_glm:badScenario', ...
             ['Unrecognized scenario "%s". Valid scenarios are: ' ...
diff --git a/analysis/matlab/tdcs_lme.m b/analysis/matlab/tdcs_lme.m
index daef865..923ac4b 100644
--- a/analysis/matlab/tdcs_lme.m
+++ b/analysis/matlab/tdcs_lme.m
@@ -36,6 +36,8 @@ A = anova(lme);
 L.lme = lme;
 L.nSubjects = numel(unique(T.subject));
 L.nObs = height(T);
+L.maxDayA = max(T.day(T.group == cfg.anchorLow));    % last day Box-A2 has data
+L.maxDayB = max(T.day(T.group == cfg.anchorHigh));   % last day Box-B2 has data
 L.interaction = localTerm(C, A, 'day:tDCS');
 L.day = localTerm(C, A, 'day');
 L.tDCS = localTerm(C, A, 'tDCS');
diff --git a/analysis/matlab/tdcs_lme_report.m b/analysis/matlab/tdcs_lme_report.m
index 14401aa..1a0e993 100644
--- a/analysis/matlab/tdcs_lme_report.m
+++ b/analysis/matlab/tdcs_lme_report.m
@@ -12,8 +12,16 @@ s = [s sprintf('%s\n', bar)];
 s = [s sprintf('model: success ~ day * tDCS + (1|subject)   [tDCS: %s = 1 vs %s = 0]\n', ...
     cfg.anchorHigh, cfg.anchorLow)];
 s = [s sprintf('N = %d subjects, %d sessions\n', L.nSubjects, L.nObs)];
+s = [s sprintf('day coverage: Box-B2 0..%d, Box-A2 0..%d\n', L.maxDayB, L.maxDayA)];
 s = [s sprintf('(day is raw and 0-indexed: our day 0 = the paper''s "Day 1", so the tDCS\n')];
-s = [s sprintf(' main effect below is the group difference on Day 1 -- comparable to the paper.)\n\n')];
+s = [s sprintf(' main effect below is the group difference on Day 1 -- comparable to the paper.)\n')];
+if abs(L.maxDayB - L.maxDayA) > 2
+    s = [s sprintf(['** WARNING: the groups'' day coverage is UNEQUAL (differ by %d days). The\n' ...
+        '   interaction/slope over this window EXTRAPOLATES the shorter group''s line and is\n' ...
+        '   CONFOUNDED -- prefer the _d0_%d window (both groups have data throughout). **\n'], ...
+        abs(L.maxDayB - L.maxDayA), min(L.maxDayB, L.maxDayA))];
+end
+s = [s sprintf('\n')];
 
 s = [s sprintf('%-27s %-14s %-13s %s\n', 'effect', 't (df)', 'F (df1)', 'p')];
 s = [s sprintf('%s\n', repmat('-', 1, 70))];
diff --git a/analysis/matlab/tdcs_scenario_data.m b/analysis/matlab/tdcs_scenario_data.m
index 4e605ad..6daea18 100644
--- a/analysis/matlab/tdcs_scenario_data.m
+++ b/analysis/matlab/tdcs_scenario_data.m
@@ -92,7 +92,7 @@ meta.isUnmerged = strcmp(mergeKey, 'unmerged');
 end
 
 function [mergeKey, maxDay] = localParseScenario(scenario)
-%LOCALPARSESCENARIO Split "<mergeKey>_full" / "<mergeKey>_d0_10" apart.
+%LOCALPARSESCENARIO Split "<mergeKey>_full" / "_d0_10" / "_d0_13" apart.
 scenario = char(scenario);
 if endsWith(scenario, '_full')
     maxDay = 26;
@@ -100,9 +100,12 @@ if endsWith(scenario, '_full')
 elseif endsWith(scenario, '_d0_10')
     maxDay = 10;
     mergeKey = scenario(1:end - numel('_d0_10'));
+elseif endsWith(scenario, '_d0_13')
+    maxDay = 13;   % day 13 = last day Box-A2 has data (fair A2-vs-B2 window)
+    mergeKey = scenario(1:end - numel('_d0_13'));
 else
     error('tdcs_scenario_data:badScenario', ...
-        'Scenario "%s" does not end in "_full" or "_d0_10".', scenario);
+        'Scenario "%s" does not end in "_full", "_d0_10", or "_d0_13".', scenario);
 end
 
 if ~ismember(mergeKey, {'unmerged', 'mergeA2', 'mergeB2'})
diff --git a/analysis/matlab/tests/tLme.m b/analysis/matlab/tests/tLme.m
index 686fc1f..16928d0 100644
--- a/analysis/matlab/tests/tLme.m
+++ b/analysis/matlab/tests/tLme.m
@@ -59,5 +59,21 @@ classdef tLme < matlab.unittest.TestCase
             testCase.verifyError(@() tdcs_glm('lme_nonsense'), 'tdcs_glm:badScenario');
         end
 
+        function testD0_13WindowFairCoverageAndParallel(testCase)
+            % The 0-13 window caps both anchor groups at day 13 (equal coverage),
+            % unlike the full window where Box-A2 stops at 13 but Box-B2 runs on.
+            cfg = tdcs_config();
+            Lfull = tdcs_lme(tdcs_scenario_data('mergeA2_full'), cfg);
+            L13 = tdcs_lme(tdcs_scenario_data('mergeA2_d0_13'), cfg);
+            % Full window: unequal coverage (A2 ends ~13, B2 runs later).
+            testCase.verifyGreaterThan(abs(Lfull.maxDayB - Lfull.maxDayA), 2);
+            % 0-13 window: equal coverage and the (truncation-driven) interaction
+            % is no longer significant -- the fair-window result.
+            testCase.verifyLessThanOrEqual(L13.maxDayA, 13);
+            testCase.verifyLessThanOrEqual(L13.maxDayB, 13);
+            testCase.verifyGreaterThan(L13.interaction.p, 0.05);
+            testCase.verifyLessThan(L13.day.p, 1e-6);   % learning still strong
+        end
+
     end
 end