Minitab Quartiles

[macro90]

Using Minitab on the following data

y: 6 4 9 3 2 4 11 8 6 7

in order y: 2 3 4 4 6 6 7 8 9 11

the first quartile (Q1) is 3.750 and the third quartile (Q3) is 8.250. How were these obtained? I thought Q1 was 3.5 and Q3 was 7.5.

[JohnM]

Unfortunately there are at least half a dozen ways to compute quartiles / percentiles, depending on what you use - some feel it should be an actual member of the data set, others feel it should be some other "boundary" based on whatever algorithm they prefer - yet again, some software programs compute percentiles on the assumption that the data come from a normal distribution...

Bottom line, you'll need to check the Help to see which method Minitab uses. FYI, for both Statgraphics and Excel, Q1=4 and Q3=8.

[quark]

Te page below lists common methods for calculating quartiles, including Minitab's.

http://mathworld.wolfram.com/Quartile.html