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ITEM-TOTAL CORRELATIONS ON EXCEL
Warren King
Department of Psychology, University of Cape Town
1.) Copy your data from the reliability calculator excel sheet
and put it in a new, blank excel
workbook. This is because the reliability calculator has
formulae built into it, and doing
additional calculations there might well interfere with the
formulae that are already in there.
2.) Your data in the new, blank workbook should be set out in
the same way as the reliability
calculator - i.e., each question gets a column, and each
respondent/participant gets a row. See
the screencap below under point (3), which shows each question
(Q1-12) in a column, and each
participant (P1-12) in a row.
3.) You then need to get the sum total scores of each person on
your questionnaire - put these
in a new column in excel. To get sum total scores, you simply
add up a person's scores on each
item. This can be done easily using the =SUM excel formula to
get the sum of the first person's
scores. See the screencap below, which shows me getting the sum
scores for participant 1 (P1).
Note that I have summed the scores from left to right in Excel,
i.e., I am summing cell B2 to cell
M2.
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4.) After you have the sum of the first person's scores, you can
autofill down to get the sum
scores for every person. To do this, move the cursor over bottom
right hand corner of the cell
with the sum score in it, and it will become a small black
cross. Drag that down until the last
person's row, and all the sum scores will be automatically
calculated. In the screencap below,
you can see I now have all the sum total scores for the
participants.
5.) You now have the total scores of everyone on your
questionnaire. To do an item-total
correlation for, e.g., Item 1, go to a blank cell and type in
=CORREL or =PEARSON, and then a
bracket (a round bracket). Excel will ask you for array 1 and
array 2. To input the data for array
1, simply highlight the column that has all the persons' scores
for Item 1, and to input the data
for array 2, simply highlight the column which has the sum total
scores in. In the screencap
below, you can see that I have entered as Array 1 the column of
scores for Q1, and Array 2 is
the sum total scores. This gives me an item-total correlation
for Item 1 of .92 very good.
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6.) Repeat Step 5 above for all your test items, but each time
Array 1 will change to the column
containing the scores on the individual test questions, e.g.,
the column with scores on Item 2,
Item 3, etc. Array 2 always stays as the sum total scores. In
the screencap below I have
completed the item-total correlations for all the items. It is
clear that Q2 and Q8 have very low
item-total correlations. When I go back to the reliability
calculator and delete these two items,
overall alpha goes up from .92 to .96 so it is clear that these
two test items are bringing down
the reliability of this test quite a bit.
