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Session 1, Part D:
Bias in Sampling

In This Part: Population and Sample | Random Sampling

Random sampling is a way to remove bias in sample selection. For example, to pick a random sample of 20 people out of a population of a 1,000, you might put all 1,000 names in a hat, then draw 20 of them. Random sampling attempts to reduce bias in sample selection, since every member of the population has an equal chance of being selected. Note 5

Here are 60 circles. Can you select five circles that best represent the size of all the circles? (The average size of the five circles should equal the average size of all the circles).

Print this page.

Then look at the picture for no longer than 20 seconds. Mark the five circles you choose. Use the scale on the picture to measure the diameter of those five circles. Find the average diameter of your sample.

The average diameter of all 60 circles is 1 unit. How close to that is your sample?

(Note that a computer, selecting any five of the 60 circles randomly, might generate average diameters ranging from as small as 0.5 units to as large as 2.2 units.)

 

Problem D2

Solution  

Can you think of any circumstances in which it would be difficult or impossible to select a simple random sample?

 
 

You may have noticed that each of the problems you looked at in this session began with a question. Providing answers to questions like these is the goal of statistics. But sometimes, the variation in our data makes it difficult to answer statistical questions.

In order to identify any patterns present in the variation, we must analyze our data by organizing and summarizing it. Once this analysis is complete, we can interpret the data to answer our questions. In later sessions, we will look at the analysis and interpretation components in more detail.

It's also important to remember that when you conduct a statistical investigation, the question you pose is designed to investigate a group ("the population"). The results of an investigation involving a sample are frequently used to draw conclusions about the entire population. If an attempt is made to include every individual from the population in a sample, then the investigation is called a census.

 

Problem D3

Solution  

Why is a census still considered a sample?

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