Quiz5ch11&12 Flashcards
What is the difference between a sample and a census?
A sample is a subset of all members of a population and a census is data from about every member of the population
What are the seven steps in devloping a sample plan?
1) define target population, 2)choose data collection method, 3) select sample frame, 4)obtain sample, 5) determine sample size, 6) select sample units, 7)Conduct fieldwork
What is one thing you should always do when defining populaion of interest?
Create screening questions to eliminate individuals who do not belong
What is a sampling frame?
A list of elements from which units to be sampled are selected
What are some examples of sampling frames?
Telephone book, customer list, random-digit dialing
What are the two major sampling methods?
Probability and nonprobability samples
What is a probabilty sample?
A known, nonzero probabilty of selection
What are four types of probability samples?
Systematic, stratified, cluster, simple random
What is a nonprobability sample?
Elements selected in a nonrandom manner (convenience)
What are four types of nonprobability samples?
Convenience, snowball, judgment, quota
What are three advantages of a probabilty sample?
Information from a representative cross-section, sampling error can be computed, results are projectable to the total population
What are errors that results when the same sample is not perfectly representative of the population?
Sampling Error
What is the formula for computing the simple random probabilty of selection?
Sample size / Population size
What sampling uses fixed skip intervals to draw elements from a numbered population?
Systematic Sampling
What is the formula for a skip interval?
Population Size / Sample Size
What are the steps in implementing a properly stratified sample?
1) Identifiy salient demographic or classification factors to correlate behavior of interest, 2) determine what % of pop. Fall into subgroups, 3) Select separate simple random sample
What are the two basic steps in cluster sampling?
1) Population of interest is divided into mutually exclusive subsets, 2) a random sample of subsets is selected
What is a nonprobabilty sample used to select additional respondents on the basis of referrals?
Snowball samples
What are general rules about determining sample size?
Larger sample-smaller error, larger sample cost more on linerar basis, before trying to determine size of sample you must decide confidence interval
What happens as sample size gets smaller?
Sampling error increases
What do you need to calculate sample size?
1) an estimate of standard deviation, 2) acceptable level of sampling error, 3) confidence interval
What is the distribution of a large number of sample means or sample proportions will approximate a normal distribution called?
Central Limit Theorem
What are four important characteristics of a normal distribution?
1) bell shaped and one mode, 2) symmetrical about the mean, 3) defined by its mean and standard deviation, 4) area =1
What is the formula for z score?
(Value of variable - mean of variable) / standard deviation
What are the percentages for the area under the curve for z values 1,2,3?
68%, 95%, 99%
What is the formula to calculate the standard error of the mean (standard deviation)?
=Standard deviation / SQRT(n)
What are the steps in calculating confidence interval?
1) calculate mean, 2)estimate standard deviation, 3) estimate standard error, 4) estimate z value, 5) calculate
What are four methods used to deal with the estimate of population standard deviation?
1) use results from prior survey, 2) conduct pilot survey, 3) use secondary research, 4) Use judgement
What is the formula for calculating sample size based on a proportion or precentage?
n = (Z^2 * (P(1-P))) / E^2
What is the type of error saying there is a differnce when there is no difference?
Type 1 Error
What is the probability of not making a Type 2 Error?
Statistical Power
What is the error of saying there is no difference when there is a difference
Type 2 Error
What does the statistical power of standard formulas for calculating sample size all assume a power of what?
50%
What are the z values for a 90%, 95%, and 99% confidence interval?
1.645, 1.96, 2.575
