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Definition . A SAMPLE is a subset of the population.

Definition . A SAMPLE is a subset of the population. We should now consider the first component of an experimental study, c ollecting data The textbook lists four protocols for performing data collection. Two are passive (you just look), two are active (you have to do something ).

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Definition . A SAMPLE is a subset of the population.

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  1. Definition. A SAMPLE is a subset of the population. We should now consider the first component of an experimental study, collecting data The textbook lists four protocols for performing data collection. Two are passive (you just look), two are active (you have to do something). Here they are: • From a published source • Observational • Survey • Designed experiment

  2. Each of the two active protocols may apply to the entire population (a census) or a subset of it (a sample.) All four protocols have some drawbacks: • Published source is only as reliable as the source is • Observational the observer may affect the experiment • Survey has two main drawbacks: selection bias and non response bias • Designed experiment design error.

  3. Whenever the experimental study is not a census the possibility of a sampling error creeps in. Choosing an appropriate sample requires a lot of care and theoretical considerations. The goal is to establish a sampling procedure that maximizes the chance of getting a sample which is as representative of the population of interest as possible. The solution usually is to collect a random sample, that is a sample so chosen that every member of the population has an equal chance of being selected.

  4. Type of Data Whether applied to an entire population or to a sample, “collecting data” simply means recording the value(s) of some variable(s) for each experimental unit. Often the values obtained are not numerical, for example • Favorite cuisine (american, chinese, italian, philippino, polish, puertorican) • Opinion poll (strongly disagree, disagree, no opinion, agree, strongly agree) • Annual Income (Low, Medium, High)

  5. Even though these values could be expressed as “numbers” (for example • Foods (1 = american, …, 6 = puertorican) • Opinion (on a scale from 1 to 5, … ) • Annual income (1=low, 2=medium, 3=high) ) the values are not truly numbers, you cannot do any arithmetic on them. We say that such variables are qualitative All other variables, those whose measures are truly numerical (your weight in pounds, you IQ, your SAT score, the width of your outstretched palm in inches)

  6. Are said to be quantitative Now check your notes for exercises we have worked in class, and do the assigned homework. End of Show

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