My first question to you is why didn't you include the two groups
as a second between-subject factor? You would then have a
2 (BS-groups) x 3 (BS-proficiency levels) x 3 (WS-sentence type).
What you would be interested in seeing is whether there
are significant interactions between groups and proficiency
levels. This would reveal whether the effect of proficiency
is constant or different across levels of group. This shows
whether proficiency is different in the two groups as well as
the nature of that effect.
As for the reviewer's comment, what you could do is rank
order participants on proficiency level within the two groups
and then match the rank 1 (lowest proficiency score) in
group 1 with the person with rank 1 (lowest proficiency score)
in group 2. Continue this for all ranks. Matching on proficiency
score should cause a significant
positive correlation to exist
between the two groups. Perform a correlated groups t-test to
determine whether the difference in mean proficiency scores is
statistically significant (i.e., the two groups differ on mean proficiency
after taking into account systematic variance to subject differences)
or not significant (i.e., the two groups are statistically equivalent).
To do this you would have to restructure the data such that
matched pairs are now the unit of analysis, such as the following:
Pair Group1 Group2
1 Rank01 Rank01
2 Rank02 Rank02
....
N RankN RankN
Where Group1 contains the ordering of the proficiency score
(Rank01 = lowest proficiency score -- do not use the rank value)
and Group 2 contains the matched value in Group 2 (i.e.,
the lowest proficiency
score in Group1 is matched with the
lowest proficiency score in Group2 and so on).
I think that the reviewer's point is that you need to control for
proficiency differences between the two groups. I think that
this could be done by including group as a between-subjects
factor which would make is a 3-way "mixed" design. Another
way of controlling for proficiency would be to have a 2-way
design consisting of Group (2-levels) x Sentence Type (3 levels)
and using proficiency as a covariate (i.e., statistically equating
the two groups on proficiency).
There probably are other ways of controlling/taking into account
proficiency effects and other people on SPSS may chime in on this.
-Mike Palij
New York University
[hidden email] ----- Original Message -----
From: "eva9" <
[hidden email]>
To: <
[hidden email]>
Sent: Tuesday, July 19, 2011 4:12 AM
Subject: Matching proficiency levels shown by statistics
>I have conducted a language acquisition study for an article with two groups
> using the same test sentences. First I made them do an independent English
> test to categorize the students into 3 levels (low-mid-high) of proficiency.
> There were 3 types of sentences, 8 test sentences for each type. Then I
> conducted a factorial ANOVA for both groups, the 3 levels as between subject
> factor and type of sentences as within subjects factor (3) in order to see
> if there are significant differences among their correct production of the
> three types.
> Now the reviewer tells me that I should match the proficiency level of the
> two groups [b]to be shown by statistics [/b]in order to be able to talk
> about their production (make sure that both groups are at comparable levels
> of proficiency in English). My question is HOW?
>
Could somebody help me? As you see I am a complete beginner in the field of
> statistics, i would need clear instructions.
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