Wednesday, December 05, 2007

The Seven Point Scale

For some time now I've noticed the scoring things on a scale of 1 to 7 seems to be a good way of evaluating some analogue or continuous phenomenon. I was first introduced to 7 point scales by John Ousterhout (who's best known as the Tcl instigator). John likes to use a 7 point scale to evaluate interviewees as follows:

1Worst candidate imaginable. I will quit if you hire them.
2Very negative. Will argue strongly against hiring.
4Totally ambivalent about this candidate.
6Enthusiastic. Will argue strongly to hire.
7Best candidate imaginable. I will quit if you don't hire them.

And we used to look for candidates with 5 and above votes from all interviewers and at least one 6. We did once have someone vote 7 on a candidate, but it's very rare to see 1 or 7.

Turns out that seven point scales are not that uncommon. Kinsey used one in defining types of sexuality:

1Exclusively heterosexual.
2Predominantly heterosexual, only incidentally homosexual.
3Predominantly heterosexual, but more than incidentally homosexual.
4Equally heterosexual and homosexual.
5Predominantly homosexual, but more than incidentally heterosexual
6Predominantly homosexual, only incidentally heterosexual
7Exclusively homosexual.

And there's actually research into the accuracy of 7 points scales. See for example this report that indicates that 7 point scales give as much information as 10 point scales when rating happiness. And here's another paper recommending seven point scales for measurement. And then there's the Likert scale which has been in use since the 1930s which has 5 and 7 point variants.

Seven point scales are neat because they have a clear middle point and between the middle and end points there are just two choices. That gives them to capture variations in opinions without presenting too many choices (leading to vacillation) or too few (meaning that too much data is lost). I'm using them lots of different places: most recently in the votes on books I've recently read.



Blogger Michael said...

The IB program (similar to AP in most US high schools) also uses a 7 point scale for grading.

12:10 AM  
Blogger Carole said...

Normally, I would disagree with the usefulness of an odd-number scale. The middle point is often appealing to those that aren't sure either way. It allows people to avoid thinking to hard about what they really think or feel, and just choose the middle point.

That said, I doubt your average person would say they were equally hetero- and homo- sexual, so perhaps the odd-number scale is more appropriate when the middle point is not the most socially acceptable or mentally laziest option.

2:47 PM  

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