Math 127B, Notes for lecture 8
Mrinal Raghupathi
Wednesday, February 9th, 2011
Administrivia
The topics to be discussed today are weighting, halfsamples and probability samples. The material
is based on chapter 22.
Reminders
1. Read chapter 23.1 – 23.2 for Monday. There is an online quiz posted due by Monday 8 am.
2. Homework 4 is due Thursday, 2/10 in recitation.
3. There will be a quiz in recitation over chapter 21.
1
Weighting the sample
A survey of undergrads is conducted to ﬁnd out how the average number of credit hours taken by
undergraduates each semester. A sample of 100 undergraduates reveals the following ﬁgures
Credit hours
Class
Number
Average
Total
Senior
21
12
12
×
21 = 252
Junior
28
14
14
×
28 = 392
Sophomore
26
13
13
×
26 = 338
Freshmen
25
15
15
×
25 = 375
The university registrar informs us each of the four classes has an equal representation in the
university and from this we see that the seniors are underrepresented in our sample. To compensate
for this we will assign a weight to each group.
What is a weight? A weight is a number that we will assign to each group that will compensate
for their size relative to the population. A weight of 1 should be assigned to a group that has a size
1
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View Full Documentequal to its proportion in the population. Underrepresented groups should be assigned weights
larger than 1. A simple way to get a weight is to compare the ratio of the size of the group in
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 Spring '11
 Staff
 Statistics, Probability, 10%, Simple random sample, 0.43 hours

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