Every term the series leans on, in alphabetical order, each with a link to the part that introduces it. Where an example helps, it comes from the teacher's study of the revision guide that the whole series uses. If a word in a methods section is unfamiliar, it should be here.

Adjusting for, controlling for
Adding a known factor such as which school or which topic to the regression so that its variation is removed from the noise before the guide's effect is judged. The regression form of stratifying. Part 13
Alternative hypothesis
The claim itself, once the null has been rejected: the guide changes the mark. One-sided if it names a direction, two-sided if it does not. Part 3
ANOVA, analysis of variance
One test for whether any of three or more group means differ, such as the guide, the one-page revision sheet and neither, before asking which. Followed by corrected pairwise comparisons. Kruskal-Wallis is its rank version. Part 12
Bayes factor
How many times more probable the data are under one hypothesis than another, each averaged over its prior. Can favour the null, which a p-value cannot. Sensitive to the priors. Part 15
Benjamini-Hochberg, false discovery rate
A correction for very many tests that limits the expected fraction of reported findings that are false, rather than the chance of any false finding at all. The right correction for a thousand comparisons, not for six. Part 7
Best of k sittings
The chance that a pupil allowed k sittings of the exam passes at least once, which is the figure a school that counts resits in its pass rate is reporting. Estimated from n recorded sittings by pupils at the same standard, c of which were passes, as one minus C(n − c, k) / C(n, k), not by plugging a single-sitting rate into a formula. Part 14
Blinding
Marking papers without knowing which pupils had the guide, enforced by removing the names and shuffling the pile rather than by willpower. Removes the marker's expectations from the marks. Part 9
Bonferroni correction
Divide the 0.05 line by the number of tests, or multiply each p-value by it. Holds the family-wise error rate at 5 per cent, at some cost in power. Part 7
Bootstrap
Building a confidence interval by resampling the pupils you have, with replacement, thousands of times, and reading the interval off the spread of the results. Stratified when the resampling stays inside each cell of the design. Part 6
Bootstrap test
A test that shifts each group onto the common mean, so the null is true, then resamples to see how often the real gap appears. Tests equal means while letting the spreads differ. Part 11
Box plot
A picture of a middle and a spread: the box is the middle half of the marks, the line is the median, the whiskers reach the rest, and lone dots are outliers. Part 2
Cell
One combination of everything the design controls: this school, this condition. School A with the guide is one cell, and the six pupils in it are its replicates. Part 1
Chi-square test
The large-sample approximation to Fisher's exact test for a table of counts. Needs an expected count of five or more in every cell of the table. Part 12
Cliff's delta
An effect size with no units: how often a random pupil with the guide outscores a random pupil without, minus the reverse. Part 6
Clopper-Pearson interval
The exact confidence interval for a rate, found by asking which true rates would make the observed count unsurprising. Never wrong, slightly wide. Part 2
Cohen's d
An effect size measured in units of the pupil-to-pupil spread: the gap divided by the standard deviation. Roughly, 0.2 is small, 0.5 medium, 0.8 large. Part 6
Cohen's kappa, weighted kappa, intraclass correlation
Measures of how closely two markers, or one marker on two occasions, agree, after discounting the agreement chance alone would give. Above 0.8 is excellent, and the twelve-school trial's re-mark gave 0.973. Part 9
Condition
A setting the study deliberately varies and compares: with the guide and without, the deluxe guide and the plain one. Part 1
Confidence interval
The range of effects the data cannot rule out. A 95 per cent interval comes from a procedure that captures the true effect in 95 of every 100 studies. Its width is what the study learned. Part 6
Confirmatory analysis
The phase in which the plan is frozen, nothing changes, and pilot data are excluded. The tests here are the ones that count. Part 10
Confound
Anything that varied along with the condition and could explain the gap instead: a different exam paper, a stronger intake, one group sitting the exam in the morning and the other in the afternoon. Randomisation is the cure. Part 9
Correlation, Pearson's r, Spearman's rho
How consistently two variables, such as minutes of revision and exam mark, move together, from −1 to +1. Pearson measures a straight line, Spearman uses ranks. A third variable moving both is the usual reason they correlate. Part 13
Credible interval
The range holding 95 per cent of the posterior. It means what people think a confidence interval means: given the data and the prior, the effect is probably in there. Part 15
Effect size
How big the difference is, in the outcome's own units where it has them: marks, percentage points, a multiplier. Not the p-value. Part 6
Equivalence test, TOST
The test that can support "no meaningful difference": two one-sided tests against a bound set in advance, or equivalently checking that the 90 per cent interval fits inside the band. Part 8
Exchangeable
The permutation test's one assumption: under the null, any pupil could have carried either label. True when the names were drawn from a hat, without regard to how the pupils would score. Part 4
Exploratory analysis
Questions asked after seeing the data, or during a pilot. Reported uncorrected, labelled as such, and never presented as a claim that cleared the bar. Part 7, Part 10
Family, family-wise error rate
A set of hypotheses tested together, and the chance that at least one of them produces a false positive. Six tests at 0.05 give a family-wise rate of 26 per cent. Part 7
Fisher's exact test
The permutation test for counts in a two-by-two table, such as School C's 16 of 18 passing against 5 of 18. Called exact because every possible deal is enumerated. Part 5
Freeze
The moment after which the hypotheses, design, sample size, outcomes, exclusion rules and analysis code stop changing, and before which no pupil whose mark will count has been given the guide. Part 10
Garden of forking paths
The sequence of reasonable analytic choices that, made after seeing the data, lead an honest analyst to a significant result in noise. Fixed by deciding the path first. Part 10
Geometric mean
The average for things that multiply: average the logarithms and undo the log. The right middle for ratios and multipliers, such as how many times longer the pupils with the guide revised. Part 2
Holm correction
A step-down version of Bonferroni: sort the p-values, compare the smallest to 0.05 divided by the number of tests, the next to 0.05 divided by one fewer, and stop at the first failure. Same protection, less waste. Part 7
Hypothesis
A bet with four parts: what is compared, on which outcome, in which direction, over what scope. Written before the data, or it is not a bet. Part 3
Intention to treat
Every pupil counts in the condition they were assigned to, including pupils who never opened the guide and pupils who missed part of the fortnight. Excluding them flatters the condition that lost the most pupils, because the ones who drop out are rarely the ones who would have scored well. Part 9
Interaction
A term in a regression for whether one effect depends on another factor: the guide's gap for pupils resitting the exam minus its gap for first-timers. Needs several times the data of a plain effect. P5 was one. Part 13
Interquartile range
The stretch between the value a quarter of the way up the sorted marks and the value three quarters of the way up: the middle half. The spread that ignores outliers. Part 2
Jackknife
Leave each observation out in turn and recompute. Shows which single pupil is doing the work. The bootstrap's older relative. Part 11
Linear regression, slope, r²
The line closest to the points. The slope is the effect per unit of input, so many marks per extra minute of revision, and r² is the share of the outcome's variation the line accounts for. With a yes-or-no input, such as guide or no guide, it is the t-test. Part 13
Mann-Whitney test
A permutation test on ranks rather than raw marks. Its natural effect size is Cliff's delta. Part 4
Mark scheme
The fixed list of what earns credit on each question, and how much, against which every paper is marked. Written before the exam, or it drifts. Part 1, Part 9
McNemar's test
The test for two exam papers, or two conditions, sat by the same pupils with a pass or fail each: only the pupils who passed one and failed the other count, and the question is whether the split of those is a coin toss. Part 14
Mean
Add the values, divide by how many. The total shared out equally, and pulled hard by any extreme value. Part 2
Median
The middle value once sorted. Half the pupils are below it. Robust to outliers, and the default for tutoring bills, minutes of revision and anything with a long tail. Part 2
Mixed-effects model
A regression in which some factors, such as which school or which topic, are treated as random draws from a population and given their own baselines, while the effect of the guide is fixed. The twelve-school trial's cross-check. Part 13
Multiple comparisons
The problem that asking several questions of noise gives noise several chances to answer one. Handled by a correction across the family. Part 7
Newcombe's method
A confidence interval for the difference between two rates, built from the Wilson interval of each. The honest interval for "16 of 18 against 5 of 18". Part 2
Null distribution
The hill of gaps that shuffling produces: what the test statistic looks like in a world where the null is true. The p-value is the share of it beyond the observed value. Part 4
Null hypothesis
The shadow of the claim: the comparison makes no difference, so the labels could be swapped. It is what gets tested, because only it says exactly what the data should look like. Part 3
Observation
One measurement of the thing being studied, start to finish, producing one number: one pupil sitting the exam once and getting a mark. The unit that sample sizes count, and because every observation here is a pupil, the series says "pupil" for short. Part 1
Observational study
A study whose conditions were chosen or simply observed rather than drawn by lot, so that other things may have travelled with them. Comparing pupils whose parents bought the guide with pupils whose parents did not would be one. Part 9
Outcome measure
The number written down at the end of each observation: a mark out of 100, a pass or fail, the minutes a pupil spent revising. Part 1
Paired bootstrap, paired t-test
For two conditions measured on the same topics, or the same pupils: analyse the differences, one per pair, so that the pair's shared difficulty cancels. Turned 0.028 into 0.00001 on the same numbers. Part 11, Part 12
Permutation test
Shuffle the condition labels, recompute the gap, repeat thousands of times, and count how often shuffling alone beats the real gap. Assumes only exchangeability. Stratified when the shuffle stays inside each cell. Part 4
Post hoc
Decided after the results were in view. Not a sin, but a label a write-up owes its reader. Part 10
Power, power analysis
The chance a study will detect an effect of a given size if it exists, usually aimed at 80 per cent. Power analysis works the calculation backwards to say how many pupils are needed. Part 8
Preregistration
Depositing the frozen plan with a registry such as OSF or AsPredicted before the first pupil is given the guide, so that a third party's timestamp proves when the choices were made. Not peer review, not a publication route. Part 10
Pre-specified
Written down, with all four ingredients and the test to be used, before the analysis. Weaker than preregistered only in that nobody else witnessed the date. Part 3
Primary and secondary contrasts
The headline bets, corrected as a family, and the follow-up questions asked to understand them, reported uncorrected and labelled exploratory. Part 3
Prior, likelihood, posterior
What you believed before the data, what the data say at each possible value, and what you believe after: Bayes' theorem multiplies the first two to get the third. Part 15
p-value
The probability, if the null hypothesis were true, of a result at least as extreme as the one observed. Not the probability that the null is true, and not a measure of how big the effect is. Part 5
Region of practical equivalence (ROPE)
Part 8's band of no meaningful effect, read as the share of the posterior that falls inside it. A probability instead of a verdict. Part 15
Registered Report
The plan is peer-reviewed before the data are collected. If accepted in principle, the report is published whatever the results, provided the protocol was followed. Part 10
Replicate, replication
A repeat of the same observation with nothing changed except the pupil, to measure the noise: the six pupils in one cell are six replicates. Not to be confused with replicability, which is someone else reaching your conclusion with new data. Part 1, Part 9
Reproducible
Someone with your data and your analysis gets your numbers. Guaranteed by publishing the anonymised marks, the mark scheme and the analysis script. Part 9
Results-blind review
The manuscript is reviewed with the results redacted, after the data exist. Removes the reviewers' outcome bias, not the author's. Part 10
Sample size, n
How many pupils sit behind a number. The single most important thing to read beside any average. Part 1
Selection effect
A bias created by which pupils are kept: a pass rate counted only among the pupils who turned up looks better than one counted among everyone who was entered. Part 9
Shrinkage
What an informative prior does to a small-study estimate: pulls it toward what was expected, in proportion to how noisy the estimate is. It pulled the twenty-four pupils' gap of 11.9 marks to 8.4. Usually wise, always to be declared. Part 15
Smallest effect size of interest
The bound, chosen before the data, inside which a difference does not matter. What "no meaningful effect" means, with a number in it. Part 8
Standard deviation
The typical distance of a pupil's mark from the mean. About 15 marks for the twenty-four pupils. Pulled by outliers, like the mean. Part 2
Standard error
How much an average moves from sample to sample: the standard deviation divided by the square root of the number of pupils. The formula-based confidence interval is the estimate plus or minus about two of these. Part 6
Statistically significant
Would be surprising under the null, by the 0.05 convention. Not the same as large, useful or important. Part 5
Stratified
Done within each cell of the design: a shuffle that never moves a mark from one school to the other, a resample that draws from each cell separately. Removes the noise that the design already accounted for. Part 4, Part 6
Test statistic
The single number computed from the data that the test is about: the difference in means, the difference in medians, a count of passes. Chosen before looking. Part 4
Threats to validity
The four ways a conclusion can be wrong: internal (a confound caused the gap), external (it does not generalise to other schools), construct (the measure is not the thing), statistical (the tests or sample were inadequate). Part 9
Trimmed mean, weighted mean
The mean with the extremes dropped, and the mean in which some items count more than others. An exam total in which the questions carry different numbers of marks is a weighted mean. Part 2
t-test
The classical test for a difference in means, which computes the p-value from a formula that assumes bell-shaped noise. Agrees with the permutation test when the assumption holds. Part 4
Tukey's HSD
The classical correction for comparing every pair of groups after an ANOVA. Holm across the pairs does the same job. Part 12
Two-sided, one-sided
Whether a test counts surprise in both directions or only the expected one. Two-sided is the cautious default and the one the twelve-school trial used throughout. Part 3
Variability, noise
How much pupils given identical preparation differ from each other. The reason one number is not a finding. Part 1
Wald interval
The textbook confidence interval for a rate, which fails at small counts and near 0 or 100 per cent, where it can claim a rate of 103 per cent. Worth recognising only to distrust. Part 2
Welch's t-test
The t-test that does not assume the two groups have the same spread. The default for two independent groups. Student's version assumes equal spreads and is what most software gives you unless asked. Part 12
Wilcoxon signed-rank test
The rank version of the paired t-test. With very few pairs it has a floor: seven pairs, one per topic, cannot give p below 0.0156. Part 12
Wilson score interval
The well-behaved confidence interval for a single rate, pulled toward 50 per cent and never past 0 or 100. The sensible default. Part 2