Exam 3 Discussion
Handout Last time (back of Optional problems)
Distribution, and buffer ("trimmed mean").
Sec. 7.2, Comparing two means
See Day 40
Summary: Once we have (xbar1 - xbar2) ,
SEdiff
, and the df, our formulas pattern on the earlier ones.
SEdiff = sqrt[SE(xbar1)2
+ SE(xbar2)2 ] (The Not-equal-variances
version.)
For doing by hand: df
= smaller of (n1- 1) and (n2- 1).
From a computer: df
= complicated formula on p. 403. Produces non-integer degrees
of freedom. Very good approximation to the exact distribution, if
both sample sizes are at least 5. Unsuitable for doing by hand.
Third way of doing these; the
"pooled two-sample t-procedure ." (See
ActivStats below, or Moore p.406.)
"Equal variances assumed"--a
different formula for SEdiff , different df. If
n1= n2, the two SEdiff formulas
give the same answer. But the df's are still different). Safer
to use "Equal variances NOT assumed" as a rule.
| t-distribution procedures:
(Repeated: Optional: two-sample with pooled variance, Activstats ch. 21-3. Don't neglect the green star. This was the only technique until "recently." It's the basis for the much used Analysis of Variance, for comparing more than 2 independent groups.) No new work. Review. |
| Hand in:
More complicated problems: putting together everything... Read them over, do what you can, bring questions ? p. 400, 7.33 Math sublimina. This is a complicated design: matched pairs, then 2-sample on the differences! But notice chicks (7.35 ) was also matched pairs--weight gain =after-before--but they gave us the pre-subtracted numbers. p.410, 7.45 fitness Do b. Then Look in the back at the answers for a and b. p.422, 7.63 pasture fertilization p.423 7.67 London bus people p.425 7.72 reading biology |
Optional
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| Sievers home | Math151-Sp02/Day41.htm | 9pm | 5/8/02 |