In this video we'll take a look at how to calculate percentiles in SPSS. To
calculate percentiles, we want to go to Analyze and then Descriptive Statistics
and then select Frequencies. Here we want to move our variable, college GPA, over to
the Variable(s) box, we'll click the right arrow button, and then select Statistics.
And then here notice there's a section named Percentile values and we'll start
by selecting Quartiles. So we'll click on that, and then we'll click Continue
and then OK. Now here our output window opens and notice we have two tables, we
have Statistics, and then here we have a table named College GPA. And in this table
this just lists the frequencies for each GPA value. So notice here we had a
frequency of 1, so there was one GPA of 2.13, there was one GPA of 2.31, and so on.
But this first table, Statistics, is the table we want to focus in on here. And
notice here we have an N valid of 25 and missing of 0.
This indicates we had 25 values that were valid for a variable and 0 that
were missing.
So going back to our variable, college GPA, notice here we do have 25 values and
there are none that are missing, so that's what this output is indicating
here. And then here's what we want to focus on here, our percentiles. So notice
we have three values reported: 25, 50, and 75. Now 25 corresponds to the 25th
percentile, and that's also known as the first quartile. So this value here, the
25th percentile,
this indicates that 25% of the people had GPAs lower than this
value. So notice we see a GPA of 2.805, or if we round that, that would give us a
GPA 2.81. So this 25th percentile, or first quartile, indicates that
approximately 25% of the people had GPAs
lower than 2.81. Our next percentile is the 50th percentile. Now the 50th
percentile is also known as the second quartile. And this indicates that, notice
there's a GPA 3.21, so 50% of the people had GPA values that were less
than 3.21, and 50% had values that were greater than 3.21. So notice
that the 50th percentile, or the second quartile, divides the values exactly in
half, where we have 50% below this value of 3.21, and 50%
above that value. Now, finally, we have the 75th percentile, which is also known as
the third quartile. And this indicates that, if we round this here to two
decimal places, about 75% of the people had a GPA less than 3.74. And that also
indicates, if we look at it from the other side, that 25% of
people had GPAs greater than 3.74. And the 75th percentile once again is also
known as the third quartile. Now in case you're curious, the 50th percentile, which
we also called the second quartile, also happens to be equal to the median,
because the median divides the distribution of scores exactly in half.
So the median for this set of scores here is 3.21. Let's go ahead and
calculate a few more percentiles. So we go to Analyze, Descriptive
Statistics, and Frequencies once again. Let's click on Statistics. And then here
I'm going to deselect Quartiles, now let's go ahead and check on Percentile(s).
And here I can enter any percentile from 1 to 99. Let's go ahead and enter the
10th percentile, click Add, and then we'll also enter the 90th percentile, and click Add.
But just so you could see I could enter any kind of percentile I wanted, so if I
wanted the 11th percentile, for example, I would just enter 11 and then click Add,
and you see it there. But I don't want that one, so I'm going to select it and
click Remove. OK so we have the 10th and 90th percentiles. We'll go ahead and click
Continue. And then here I'm going to uncheck this option here, Display
frequency
tables, and then I won't get this table a second time, as I don't really need
it. So I'll uncheck that. Let's go and click OK. And then here we have our one table,
Statistics, again.
And notice that we have percentiles now of 10 and 90. So the 10th percentile
corresponds to a GPA of 2.32, with rounding. So that indicates that approximately
10% of the people here had GPAs that were less than 2.32,
which also implies that 90%, the remaining, had GPAs that were
approximately greater than 2.32. And then, finally, the 90th percentile corresponds
to a GPA of about 3.86. So that indicates that approximately 90% of the
people had GPAs lower than 3.86, which also means that approximately
10% of the people had a GPA higher than 3.6. OK that's it.
This concludes the video on calculating percentiles in SPSS. Thanks for watching.