7 common errors

Some common errors arise again and again in statistics.
Here are seven to watch out for:

unclear basepoint for graphs

A TV advert used to proudly proclaim:
"X has 25% more active ingredient"

The screen however showed just the top of four test tubes. The words may have been true, but it looked a lot more than 25% - a truthful advert?


%increases and changes in %difference

If 300 is 50% bigger than 200, is 200 50% less than 300?
In 1990 product A had 10% of market share and now it has 15%. Is that a 50% increase or a 5% increase? Of course, the market may be only half as big now, so there may be less of product A sold.
Lesson - be very careful with your language.

beware extrapolation

Interpolation - estimating unknown values between known values - OK, but extrapolating - going outside the known limits is dangerous (albeit sometimes necessary!).


statistical significant important

Real, non-random effects may nevertheless be very small

Assuming that a non-significant result means no difference is like Kate Winslett assuming she weighs nothing because there was no detectable change in the waterline of the Titanic when she jumped off.

non-significant no effect

Big effects may not be significant if sample size is low or variability high.

relationship causality

There may be a common cause, or it may simply be a fluke!

don't do too many tests!!

5% significant means will happen by chance 1 time in 20. If you do lots of tests, 1 in 20 will (on average) be 5% significant. So, if you need to do lots of tests look for better significance values.


tutorial outline

tutorial notes

7 common errors

book list

rainfall in Gheisra

two-horse races

more coin tossing!

HyperCard demo

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http://www.meandeviation.com/tutorials/stats/notes/7errors.html

Alan Dix 5/8/98