R.E.Bove 11/6/00. 251thngs
ECO 251 - THINGS THAT YOU SHOULD NEVER DO ON A STATISTICS EXAM (OR
ANYWHERE ELSE)
1. Claim that mutually exclusive events are also independent.
2. Compute
 x  x 
2
x
2
by computing
 x and squaring it. (Or computing  x   x 
2
when
is required.)
3. Compute
 xy by multiplying  x by  y .
4. Claim that a point below the 50th percentile is above the median, or that a point above the 50th
percentile is below the median.
5. Compute a median or fractile without putting the data in order or grouping it.
6. Use a formula for grouped data for ungrouped data, or use a formula for ungrouped data for
grouped data.
7. Claim that a variance ( 2 or s 2 ) or a standard deviation is negative (or that the variance or
standard deviation of x is zero unless x is a constant).
8. Claim that a sum of squares is negative.
x 2  nx 2 is negative.
a. Claim that

b. Think that because
x
2
 nx 2 can't be negative, neither can
 xy  nxy .
c. Take the square root of a negative number .
9. Claim that a probability (or a p  value ) is above 1 or negative.
10. Compute a sample statistic or a population parameter without checking the value of n or N.
11. Claim that a correlation (r ) is above 1 or below -1 or that a coefficient of determination ( R 2 )
is above 1 or negative.
12. When working with grouped data, using  fx 2 or  f x  x 2 instead of fx2 or f x  x 2 .
13. When asked for the probability of 'at least one,' give the probability of exactly one instead ( or
when asked for Px  5 giving Px  5 instead.
-------------------------------------------------------------------------------------------------------------------------------Of course, due to rounding errors or arithmetic mistakes, some of the above things will occur. If
they do, look for your error . If you can't find it, don't panic. Your answer may still be good for whole or
partial credit if you include a note on your paper that you realize that your result is impossible.