Math 127B, Notes for lecture 5
Mrinal Raghupathi
Wednesday, January 31st, 2011
Administrivia
The topics to be discussed today are the accuracy of percentages and conﬁdence intervals. Most of
the material is taken from chapter 21.1 – 21.3.
Reminders
1. Read chapter 21.3, 21.4, 21.5 for Wednesday. There is an online quiz posted due by Wednesday
8 am.
2. Homework 3 is due Thursday, 2/3 in recitation.
3. There will be a quiz in recitation over chapter 20.
1 Inference from samples
So far we have seen what happens to a sample drawn from a box with replacement. We also looked
at the correction factor for samples without replacement. We will not focus on this distinction
except in extreme situations (which will be explicitly mentioned). The moral is: The size of the
sample determines the accuracy of the estimate.
Given that we know the composition of the box, i.e., the number of 0s and 1s, we have described
ways to compute the EV and SE of percentages for a sample.
Inference attempts to reverse this process. In practice we will not know the parameter. After
all, if you did know the parameter, you wouldn’t need to conduct a survey! We illustrate our ideas
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 Spring '11
 Staff
 Math, Statistics, Approximation, Simple random sample

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