# STA 13 Lecture 3: stat 13: lecture 3

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7 Feb 2017
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1/13/17
Components of experiments
1. Control group
Use as a comparison group to test new drug
2. Placebo - an inactive drug designed to be indistinguishable from active drug
3. Placebo effect
Improvement when you “know” you are taking a drug
4. Single-blind
Control gets placebo
Subjects in both groups don’t know which they get
5. Double - blind
Both subjects and the evaluators don't know (“blind”) which drug they got
When the outcomes being measured have any degree of subjectivity, if the
person doing the measurement knows which drug the subject is getting this may
bias results
Chapter 2
Summarizing and graphing data
We use frequencies and relative frequencies to summarize and graph data
Frequency table
Summarizes data by grouping data values into categories
Relative frequency = frequency / N
Relative frequency is the proportion (fraction) of values in the data
Ex: relative frequency for never married = 18/180 = 0.1
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Ex: a discrete random variable sample of 8 families with following numbers of
children 0,1,2,2,1,2,3,4
Notice the 2 graphs have exactly the same shape. The only difference is the y-
axis
These graphs are called histograms
Histograms have 3 choices for the y-axis
1. Frequency
2. Relative frequency
3. Percent = relative frequency
All 3 graphs will have same shape
When we have data for a discrete variable (like # children) we can make a bar for every
value in the data
When we have data for a continuous random variable we have to group the data values
into a set of intervals
To make histogram for continuous data
1. Divide values into intervals
Call these classes
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2. Count number of data in each class
Counts are called frequencies
3. Draw bar for each class with height using one of the following
Frequency = number values in class
Relative frequency = (frequency) / (total number values)
Or percent = relative frequency x 100%
Example of data for a continuous variable: annual incomes for n = 20 families
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