Section 2. Ways of obtaining samples  1. Random Sample - every member of the population must have an equal chance of being selected.

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Presentation transcript:

Section 2

Ways of obtaining samples  1. Random Sample - every member of the population must have an equal chance of being selected

Sampling continued Sampling continued  2. Stratified Samples - the population is divided into subpopulations (groups inside a population that share a common characteristic), called strata and a random sample is taken from each stratum –proportional stratified sample – the size of the sample from each stratum is proportional to the size of the stratum in the population

Sampling Continued  3. Cluster Samples - obtained by selecting some of the strata and then sampling from each of these  4. Systematic Samples - from a list of members of a population, choose a starting point by chance and then select every nth element on the list

Sampling Continued  5. Samples of Convenience - a sample that already exists and is available for study; the elements in the sample are not chosen by a chance process

Some Problems with Sampling  nonresponse bias - occurs when a segment of the population is inadequately reflected in the sample  response bias - occurs when a question is phrased to elicit a certain response, or when respondents lie

Identify if the type of sampling used is random, stratified, cluster, systematic, or convenience.  1. In an effort to determine customer satisfaction, United Airlines randomly selects 50 flights during a certain week and surveys all passengers on the flights.

Identify if the type of sampling used is random, stratified, cluster, systematic, or convenience.  2. A radio station asks its listeners to call in their opinion regarding the use of American forces in peacekeeping missions.

Identify if the type of sampling used is random, stratified, cluster, systematic, or convenience.  3. In order to estimate the percentage of defects in a recent manufacturing batch, a quality control manager at Intel selects every 8th chip that comes off the assembly line starting with the 3rd, until she obtains a sample of 140 chips.

Identify if the type of sampling used is random, stratified, cluster, systematic, or convenience.  4. In an effort to identify whether an advertising campaign has been effective, a marketing firm conducts a nationwide poll by randomly selecting individuals from a list of known users of the product.

Identify if the type of sampling used is random, stratified, cluster, systematic, or convenience.  5. A school official divides the student population into five classes: freshman, sophomore, junior, senior, graduate student. The official takes a random sample from each class and asks the members’ opinions regarding student services.

Determine why the survey design is flawed.  1. A college vice president wants to conduct a study regarding student achievement of undergraduate students. He selects the first 50 students who enter the building on a given day and administers his survey.

Determine why the survey design is flawed.  2. The Village of Oak Lawn wishes to conduct a study regarding the income level of households within the village. The village manager selects 10 homes in the southwest corner of the village and sends an interviewer to the homes to ascertain the household income.

Determine why the survey design is flawed.  3. Suppose you are conducting a survey regarding students’ study habits. You obtain a list of students and obtain a simple random sample of 90 students. The survey is administered by a teacher.