1.2 Random Samples

A simple random sample of n measurements from a population is a subset of the population selected in a manner such that

a) every sample of size n from the population has an

equal chance of being selectedANDb) every member of the population has an equal chance

of being included in the sample.*Guided Exercise 3 p.12

Use a random-number table• Table 1 in the Appendix p. A1

Sampling Techniques

Assign each object a number, write numbers on cards, mix up cards, choose random cards

1)Random sampling

Another use for a random-number table is in simulation.

A simulation is a numerical facsimile or representation of a real-world phenomenon.

*Guided Exercise 4 p.14

2) Stratified sampling—the population is broken down into groups/classes (strata) and a random sample is chosen from each strata

i.e. Population: All ASFA studentsStrata: CW, DA, MS, MU, VA, TA

or7th, 8th, 9th, 10th, 11th, 12th gradesormale students, female students

Other Sampling Techniques

3) Systematic sampling—assumes the elements of the population are arranged into some natural order. Select a random starting point, then choose every kth element.

i.e. Population: People lined up to purchase concert tickets

Sample: choose every 5th person in line (with the starting person being one of the first 5 people in line)

*Easy way to draw a sample, but should not be used when the population is cyclic in nature (fabric mill/loom)

Other Sampling Techniques cont’d

4) Cluster sampling—divide demographic area into sections. Randomly choose sections or clusters. Each member of a cluster is included in the sample.

Used by government agencies and private research organizations

5) Convenience sampling—uses data or results that are conveniently or readily available.

High risk of bias

Other Sampling Techniques cont’d

p. 17 #1 –9 odd

Classwork