A hospital wants to know how a homeopathic medicine for depression performs in comparison to alternatives. They adminstered 4 treatments to 100 patients for 2 weeks and then measured their depression levels. The data, part of which are shown above, are in depression.sav.

## Data Inspection - Split Histogram

Before running any statistical test, always make sure your data make sense in the first place. In this case, a split histogram basically tells the whole story in a single chart. We don't see many SPSS users run such charts but you'll see in a minute how incredibly useful it is. The screenshots below show how to create it.

In step below, you can add a nice title to your chart. We settled for “Distribution BDI per Medicine”.

## Syntax for Split Histogram

Clicking syntax below. Running it creates our chart.

results in the***Run histograms of BDI scores for the four medicines separately.**

GRAPH

/HISTOGRAM=bdi

/PANEL ROWVAR=medicine ROWOP=CROSS

/TITLE='Distribution BDI per Medicine'.

## Result

These simple charts give a lot of insight into our data. The important points are:

- All
**distributions look plausible**. We don't see very low or high BDI scores that should be set as user missing values and the BDI scores even look reasonably normally distributed. - The medicine
**“None” results in the highest BDI scores**, indicating the worst depressive symptoms. “Pharmaceutical” results in the lowest levels of depressive illness and the other two treatments are in between. - The four histograms are roughly equally wide, suggesting BDI scores have
**roughly equal variances**over our four medicines.

## Means Table

We'll now take a more precise look at our data by running a means table. We could do so from

but the syntax is so simple that just typing it is probably faster.***Run basic means table.**

means bdi by medicine

/cells count min max mean variance.

***Note: use /cells to choose which columns you'd like in which order.**

## Result

Unsurprisingly, our table mostly confirms what we already saw in our histogram. Note (under “N”) that each medicine has 25 observations so these two variables don't contain any missing values.

So can we conclude that “Pharmaceutical” performs best and “None” performs worst? Well, for our sample we can. For our population (all people suffering from depression) we can't. The basic problem here is that
samples differ from the populations from which they are drawn.
If our four medicines perform equally well in our population, then we may still see some differences between our sample means. However, *large* sample differences are unlikely if all medicines perform equally in our population. For an outstanding explanation of this reasoning, read up on ANOVA - What Is It?

The question we'll now answer is: **are the sample means different enough** to reject the null hypothesis that the mean BDI scores in our populations are all equal?

## ANOVA - Omnibus Test and Post Hoc Tests

We often run ANOVA in 2 steps:

- we first test if
*all*means are equal. This is often called the**omnibus test**. “Omnibus” is Latin for “about everything”. - if we conclude that
*not all*means are equal, we sometimes test precisely*which*means are not equal. This involves**post hoc tests**. “Post hoc” is Latin for “after that” in which “that” refers to the omnibus test. Right?

This standard procedure suggests that you should
only run post hoc tests if the omnibus test

is “statistically significant”.
However, it could be argued that you should *always* run post hoc tests. In some fields like market research, this is pretty common. Reversely, you could argue that you should *never* use post hoc tests because the omnibus test suffices: some analysts claim that running post hoc tests is **overanalyzing** the data. Many social scientists are completely obsessed with statistical significance -because they don't understand what it *really* means- and neglect what's more interesting: effect sizes and confidence intervals.

In any case, the idea of post hoc tests is clarified best by just running them. But before doing so, let's take a quick look at the assumptions required for running ANOVA in the first place.

## ANOVA Assumptions

Our ANOVA will run fine in SPSS but we can take the results seriously only if our data satisfy 3 assumptions:

**Independent observations**often holds if each case (row of cells in SPSS) represents a unique person or other statistical unit. That is, we usually don't want more than one row of data for one person, which holds for our data;**Normally distributed variables**in the population seems reasonable if we look at the histograms we inspected earlier. Besideds, violation of the normality assumption is no real issue for larger sample sizes due to the central limit theorem.**Homogeneity**means that the population variances of BDI in each medicine group are all equal, reflected in roughly equal sample variances. Again, our split histogram suggests this is the case but we'll try and confirm this by including**Levene's test**when running our ANOVA.

## Running our ANOVA in SPSS

There's many ways to run the exact same ANOVA in SPSS. Today, we'll go for

because creates nicely detailed output.We'll briefly jump into

and before pasting our syntax.The post hoc test we'll run is Tukey’s HSD (Honestly Significant Difference), denoted as “Tukey”. We'll explain how it works when we'll discuss the output.

“Estimates of effect size” refers to partial eta squared. “Homogeneity tests” includes Levene’s test for equal variances in our output.

## SPSS Post Hoc ANOVA Syntax

Following the previous screenshots results in the syntax below. We'll run it and explain the output.

***ANOVA syntax with Post Hoc (Tukey) test, homogeneity (Levene's test) and effect size (partial eta squared).**

UNIANOVA bdi BY medicine

/METHOD=SSTYPE(3)

/INTERCEPT=INCLUDE

/POSTHOC=medicine(TUKEY)

/PRINT=ETASQ HOMOGENEITY

/CRITERIA=ALPHA(.05)

/DESIGN=medicine.

## SPSS ANOVA Output - Levene’s Test

Levene’s Test checks if the *population* variances of BDI for the four medicine groups are all equal, which is a requirement for ANOVA. As a rule of thumb,
we reject the null hypothesis if p (or “Sig.”) < 0.05.
In our case, p = 0.949 so we do *not* reject the null hypothesis of equal variances (or homogeneity). We assume the population variances are all equal so this ANOVA assumption is met by our data.

## SPSS ANOVA Output - Between Subjects Effects

The table below reports the aforementioned ANOVA omnibus test.

Our null hypothesis is that the population means are equal for all medicines adminstered. P (“Sig.”) = 0.000 -way less than 0.05- so we reject this hypothesis: the **population means are not all equal**. Some medicines result in lower mean BDI scores than other medicines.

The different medicines administered account for some 39% of the variance in the BDI scores. This is the

**effect size**as indicated by partial eta squared.

Partial Eta Squared is the Sums of Squares for medicine divided by the corrected total sums of squares (2780 / 7071 = 0.39).

Sums of Squares Error represents the variance in BDI scores

*not*accounted for by medicine. Note that + = .

## SPSS ANOVA Output - Multiple Comparisons

So far, we only concluded that our four population means being *all* equal is very unlikely. So exactly
*which* mean differs from *which* mean?
Well, the histograms and means tables we ran before our ANOVA point us in the right direction. However, we'll try and back that up with a more formal test: Tukey’s HSD as shown in the multiple comparisons table.

Right, now comparing 4 means results in (4 - 1) x 4 x 0.5 = 6 distinct comparisons, each of which is listed twice in this table. There's three ways for telling which means are likely to be different:

Statistically significant mean differences are **flagged** with an asterisk (*). For instance, the very first line tells us that “None” has a mean BDI score of 6.7 points higher than the placebo -which is quite a lot actually since BDI scores can range from 0 through 63.

As a rule of thumb, **“Sig.” < 0.05 indicates a statistically significant difference** between two means.

A **confidence interval** *not* including zero means that a zero difference between these means in the population is unlikely.

Obviously, , and result in the same conclusions.

## SPSS ANOVA - APA Reporting Post Hoc Tests

So far, so good: we ran and interpreted an ANOVA with post hoc tests. However, the tables we created don't come even close to APA standards. We can run a much better table with the CTABLES syntax below. Honestly, I'm not sure how -or even *if*- it could be created from the menu but you can hopefully reuse it after just replacing the 2 variable names.

## SPSS CTABLES Syntax for APA ANOVA Table

***APA ANOVA table - means, SD's and Bonferroni corrected pairwise t-tests.**

ctables

/table

**bdi**[s] [count 'n' f3 mean 'Mean' f3.2 stddev 'SD' f3.2] by

**medicine**[c]

/categories variables =

**medicine**total = yes position = after

/slabels position = row

/titles title = 'Table 1 - Mean BDI Scores per Medicine with Bonferroni Adjusted Pairwise T-Tests'

/comparetest type = mean style = simple merge = yes.

***Note: running this requires SPSS custom tables license.**

## Result

First off, the capitals in this table (A, B and so on) indicate which *means* differ. SPSS also flags standard deviations and sample sizes. This is **utter stupidity** because these are not compared. They always have the same flags as the means. So just ignore everything except the actual means here.

Understanding this table starts with carefully reading its footnotes. First off, “two-sided tests” refers to independent samples t-tests using 2-tailed significance.

Next, each statistically significant difference is indicated only once in this table. As indicated before, 4 means yield 6 unique pairs of means. Altogether, the table has 5 significance markers (A, B and so on). This means that only (6 - 5 =)
only 1 pair of means do *not* differ.
After some puzzling these turn out to be homeopathic versus placebo. This is the exact same conclusion we drew earlier from our pairwise comparisons (Tukey’s) table.

So that's it for now. I hope this tutorial helps you to run ANOVA with post hoc tests confidently. If you have any suggestions, please let me know by leaving a comment below.

## THIS TUTORIAL HAS 33 COMMENTS:

## By Schalk van Vuuren on June 10th, 2016

Excellent explanation of SPSS output. Well done!

## By jos reulen on June 15th, 2016

apply anova general linear model on a specific variable for four groups. now i have to select co-variates. I just add all possible co-variates and check which ones are significant. then i remove the non sinificant ones from the list of co-variates and run the analyses again.

now i want to analyse another similar parameter for comparison with the first one. is it permitted to do the same procedure resulting in different significant co-variates or should i use the same co-variates from the firts analysis ?

## By Ruben Geert van den Berg on June 16th, 2016

Dear Jos,

Your procedure is generally frowned upon because it heavily capitalizes on chance. Your final model may perform great in your sample but very poorly in the larger population or a different sample.

Your approach is like throwing 1,000 darts at a dart board, removing all missing darts and only then presenting the result. Most scientists will tell you to select a small number of covariates based on previous research and sound theoretical considerations instead of trying out many options.

However, following these guidelines, how will you ever discover anything new? Well, one really nice approach is to split your sample into two random halves. Use one half to identify your covariates. Then test your final model on the other half and see to what extent r squared holds up. This obviously requires sufficient sample size but you'll need that anyway for including many factors and covariates in your model due to shrinkage.

Hope that helps!

## By Yotam Dalal on June 30th, 2016

Is there any chance of explaining how to interpret the duncan post hoc test (Duncan's MRT)?

Thank you very much for the help so far

## By Ruben Geert van den Berg on July 1st, 2016

Hi Yotam!

There seems to be -at least approximate- consensus that Tukey's HSD as discussed in this tutorial is the best post hoc test for ANOVA we've available. I don't see why we should use a different one, neither could I find anything about Duncan's MRT in my statistics library. Correct me if I'm wrong but so far I can only conclude it had better be avoided.