- Known probability of selection
Types of Probability Samples
- Simple random sample
o A SRS of size n is taken when every possible sub set of n units in the population has same chance of being the sample
- Stratified random sample
- Cluster sample
Framework for Probability Sampling
- Sampling distribution
Simple Random Sampling
- Simple random sample with replacement
o In finite population sampling, sampling with replacement provides no additional information.
- Simple random sample without replacement
- Finite population correction
o Make this correction because with small populations, the greater our sampling fraction, n/N, the more information we have about the population and thus smaller variance.
- For large populations it is the size of the sample taken, not the percentage of population sampled that determines precision of estimator.
o If soup well stirred, you need taste only 1/2 spoonfuls, whether you have made 1 or 20 litres.
Confidence Intervals
Sample Size Estimation
- Specify tolerable error
- Find an equation
- Estimate unknown quantities
Systematic Sampling
- Is technically a form of cluster sampling
- Randomly chose first point, then take kth point
Randomization Theory Results for Simple Random Sampling
A Model for Simple Random sampling
- Joint probability distribution … supplies link between units in the sample and units not in the sample.
When Should a Simple Random sample Be used
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