Chapter 2 - Chapter 2 Samples, Good and Bad In this chapter...

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Chapter 2 Samples, Good and Bad In this chapter Bias Sampling Types of Random Sampling Bias Bias – a prejudice in one direction If a statistic is biased it will tend to overestimate or underestimate the value we are trying to predict. We can eliminate (or at least minimize) bias by using proper sampling techniques. Sampling Convenience Sampling - Uses results or data that are conveniently and readily obtained. Data from this type of sample runs risk of being severely biased and should not be generalized to the overall population (cannot use to conduct inferential statistics). Descriptive statistics can be calculated for the data however these numbers can only be used to describe the sample of volunteers. This summary of the data is not of much practical use for most questions of interest. Voluntary response sample – A common convenience sample that is selected by subjects volunteering. This kind of sample typically over-represents people with strong opinions. Examples 1. Restaurant comment cards: Who feels out restaurant comment cards? If a manager looks at the cards at the end of the day will the cards accurately portray the service that was provided at the restaurant that day? People will most likely feel out these cards if they are unhappy with their service that day. They may fill them out if they were extremely happy with their service. It is not very common for someone to feel one out and say they had average service. As with most volunteer samples people with strong opinions are much more likely to be included in your sample. Therefore, this data should never be extended to the population and used to determine the level of service at the restaurant. This does not mean that the data is meaningless. More than anything, it gives a sense of how many individuals were unhappy with their service. Obviously, 2 negative comment cards is better than 20.
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2. Medical volunteers: When testing a drug, it is very common to begin testing on volunteers because that is the only ethical option. You cannot force people to take a drug. To illustrate, suppose we have a new treatment for a certain type of cancer. Who do you think is most likely to volunteer for this experimental drug? The most likely volunteers are individuals with the most severe of cases. Individuals who have already tried traditional treatments and are out of other options. The fact that drugs are tested on a sample that is not representative of the population is problematic. Using volunteer samples is partly the reason why drugs are sometimes pulled back off
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Chapter 2 - Chapter 2 Samples, Good and Bad In this chapter...

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