Shiken: JALT Testing & Evaluation SIG Newsletter
Vol. 12 No. 2. Apr. 2008. (p. 36 - 41) [ISSN 1881-5537]
Effect size and eta squared |
 James Dean Brown University of Hawai'i at Manoa |
QUESTION:
In Chapter 6 of the 2008 book on heritage language learning that you co-edited with Kimi-Kondo Brown,
a study comparing how three different groups of informants use intersentential referencing is outlined.
On page 147 of that book, a MANOVA with a partial eta2 of .29 is outlined. There are several questions
about this statistic. What does a "partial eta" measure? Are there other forms of eta that readers should
know about? And how should one interpret a partial eta2 value of .29?
ANSWER: I will answer your question about partial eta2 in two parts.
I will start by defining and explaining eta2. Then I will circle back and do the same for partial eta2.
Eta2 can be defined as the proportion of variance associated with or accounted for by each of the main effects, interactions,
and error in an ANOVA study (see Tabachnick & Fidell, 2001, pp. 54-55, and Thompson, 2006, pp. 317-319). Formulaically, eta2,
or η2, is defined as follows:
Eta2 is most often reported for straightforward ANOVA designs that (a) are balanced (i.e., have equal cell sizes) and (b) have independent cells (i.e., different people appear in each cell). For example, in Brown (2007), I used an example ANOVA to demonstrate how to calculate power with SPSS. That was a 2 x 2 two-way ANOVA with anxiety and tension as the independent variables and trial 3 as the dependent variable (using the Anxiety 2.sav example file that comes with recent versions of the SPSS software). There were three people in each cell and the cells were independent.
Notice in Table 1 that the p values (0.90, 0.55, & 0.10) indicate that there were no significant effects (i.e., no p values below .05) for Anxiety, Tension, or their interaction. Note also that there was not sufficient power to detect such effects (i.e., the power statistics of 0.05, 0.09, & 0.37 were not above .80 in any case). All of this led me to conclude that "the study lacked sufficient power to detect any significant effects even if they exist in reality", which is reasonable given the very small sample size of 12.
| Source | SS | df | MS | F | p | eta2 | Power |
| Anxiety | 0.08 | 1 | 0.08 | 0.02 | 0.90 | 0.0012 | 0.05 |
| Tension | 2.08 | 1 | 2.08 | 0.38 | 0.55 | 0.0324 | 0.09 |
| Anxiety x Tension | 18.75 | 1 | 18.75 | 3.46 | 0.10 | 0.2919 | 0.37 |
| Error | 43.33 | 8 | 5.42 | 0.6745 | |||
| Total | 64.24 | 12 |
[ p. 36 ]
| Anxiety | Tension | M | SD | N |
| 1 | 1 | 8.67 | 3.06 | 3 |
| 2 | 7.00 | 2.65 | 3 | |
| 2 | 1 | 6.00 | 2.00 | 3 |
| 2 | 9.33 | 1.16 | 3 |
Nonetheless, even a cursory look at the means shown in Table 2 indicates that fairly large differences exist between
means and something noteworthy is going on, so a better designed replication study with a larger sample size might be
justified. Eta2 can help in interpreting the results by indicating the relative degree to which the variance that was
found in the ANOVA was associated with each of the main effects (Anxiety and Tension) and their interaction.
Eta2 values are easy to calculate. Simply add up all the sums of squares (SS), the total of which is 64.24 in the
example; then, divide the SS for each of the main effects, the interaction, and the error term by that total. The
results will be as follows:
Interpretation of these values is easiest if the decimal point is moved two places to the right in each case,
the result of which can be interpreted as percentages of variance associated with each of the main effects,
the interaction, and error. Starting with Anxiety, the value of 0.0012 indicates that a mere 0.12% of the
variance is accounted for by Anxiety, whereas Tension accounts for 3.24%, the Anxiety x Tension (A x T) interaction
accounts for a much larger 29.19%, and a whopping 67.45% is accounted for by Error. Now let's consider the A x T
interaction and Error separately in more detail.
[ p. 37 ]
The 29.19% accounted for by the A x T interaction should lead the researcher to understand that this interaction effect
is much more important than either of the individual main effects for Anxiety or Tension, a fact that, even though there
are no significant effects, may help in designing future studies and understanding why the present one did not detect
significant differences. Such an important interaction effect should lead the researcher to want to plot out that
relationship as shown in Figure 1, where we see that the Tension groups 1 (dotted line) and 2 (plain black line) do indeed
have different means but in opposite relationships for Anxiety 1 and 2. That is, the Tension 1 group is higher than the
Tension 2 group when Anxiety is 1, but the Tension 1 group is lower than the Tension 2 group when Anxiety is 2. Thus
there is a strong pattern but it is not consistent across Anxiety 1 and 2 conditions (if it were consistent, the lines
would be parallel). Thus, even with a non-significant interaction (where p = .10), the eta2 value of .2919 drew our
attention to an important interaction effect that is revealing in itself, and which may help to understand why there
were no significant main effects for Tension or Anxiety (i.e., because the interaction cancels out any such differences).
The whopping 67.45% accounted for by Error in the Table 1 indicates that more than two-thirds of the variance was not
accounted for at all in this design. This error variance may be due to unreliable variance in the study due to poor design,
other systematic variables that might be of interest (if they were operationalized and included in the study), and so forth.
All in all, eta2 values indicate not only that the interaction effect and error are causing almost 97% of the variance in
the study (67.45 + 29.19 = 96.64), but also ways to redesign the study so it will be more powerful and meaningful.
One problem with eta2 is that the magnitude of eta2 for each particular effect depends to some degree on the significance
and number of other effects in the design (Tabachnick & Fidell, 2001, p. 54). One statistic that minimizes the effects of
this issue is called partial eta2.
Partial eta2 can be defined as the ratio of variance accounted for by an effect and that effect plus its associated error
variance within an ANOVA study. Formulaically, partial eta2, or η2partial, is defined as follows:
[ p. 38 ]
In applied linguistics studies, partial eta2 is most often reported for ANOVA designs that have non-independent cells (i.e., the
same people appear in more than one cell). For example, in Brown, Hilgers, and Marsella (1991), students wrote compositions on
two different types of topics (a narrative topic and an analytic topic) which were organized into ten prompt sets. The people
who wrote on each of the ten prompt sets were different from each other (so this is also known as a between subjects effect).
In constrast, every student wrote on each of the two topic types, so these were treated as repeated measures (also known as a within
subjects effect). The cell sizes within subjects were exactly the same (which makes sense because they were the same people), whereas
the cell sizes between subjects were different to small degrees. The original results of this 10 x 2 two way repeated-measures
ANOVA for prompt sets and topic types are shown in Table 3.| Source | SS | df | MS | F | p |
| Between Subjects | |||||
| Prompt Set |
158.372 | 9 | 17.597 | 9.703 | 0.00 |
| Error |
3068.553 | 1692 | 1.814 | ||
| Within Subjects | |||||
| Topic Type | 0.344 | 1 | 0.344 | 0.194 | 0.66 |
| Prompt Set by Topic Type |
137.572 | 9 | 15.286 | 8.611 | 0.00 |
| Error |
3003.548 | 1692 | 1.775 | ||
| Source | SS | df | MS | F | p | Partial eta2 |
| Between Subjects | ||||||
| Prompt Set |
158.372 | 9 | 17.597 | 9.703 | 0.00 | 0.0490 |
| ErrorBS |
3068.553 | 1692 | 1.814 | |||
| Within Subjects | ||||||
| Topic Type | 0.344 | 1 | 0.344 | 0.194 | 0.66 | 0.0490 |
| Prompt Set by Topic Type |
137.572 | 9 | 15.286 | 8.611 | 0.00 | 0.0438 |
| ErrorWS |
3003.548 | 1692 | 1.775 | |||
[ p. 39 ]
From my present perspective (17 years later), the Brown et al (1991) study would have been strengthened by relabeling
the effects and adding partial eta2 values to the two-way repeated-measures ANOVA table as shown in Table 4. These
partial eta2 values are easy to calculate. Simply divide the SS for each effect by the SS of that effect plus the SS
for the error associated with that effect. The results will be as follows: 
In direct answer to your question, Kondo-Brown and Fukuda (2008) correctly chose to use partial eta2 because their design was a MANOVA,
which by definition involves non-independent or repeated measures. When they reported that partial eta2 was .29, that meant that the
effect for group differences in their MANOVA accounted for 29% of the group-differences plus associated error variance as explained
above. This percentage was sufficient to lead them to do univariate follow-up ANOVAs that helped them to further isolate exactly where
the significant and interesting means differences were to be found.
In recent columns, I have covered a number of issues related to the ANOVA sorts of studies including: sampling and generalizability,
sampling errors, sample size and power, and effect size and eta squared. All of these are ways to expand your thinking about ANOVA-ways
that are often ignored in applied linguistics. They have long been important to understanding ANOVA results in psychology, education,
and other fields, and we ignore them to our detriment. To paraphrase something one of my stats teachers said back in the late 1970s:
Reporting the traditional ANOVA source table (with SS, df, MS, F, and p) and discussing the associated significance levels isn't the
end of the study; it's just the beginning because we can learn much more by carefully plotting and considering the interaction
effects and doing follow up analyses like planned or post-hoc comparisons, power and effect size analyses, and so forth. I hope
I have delivered that message loud and clear.
[ p. 40 ]
Brown, J. D., Hilgers, T., & Marsella, J. (1991). Essay prompts and topics: Minimizing the effect of differences. Written Communication, 8(4), 532-555.| Where to Submit Questions: |
| Please submit questions for this column to the following address: |
|
JD Brown Department of Second Language Studies University of Hawai'i at Manoa 1890 East-West Road Honolulu, HI 96822 USA |
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