Sampling Decisionsin Your TC Program Evaluation

TCEC

Overview Why sampling? Terms Sampling methods Sample size Sample size and analysis Answers to program questions Resources Announcements

Why sampling?

Evidence from data drive the program

Census (collecting data from each entity such as stores, tenants, population) may not be possible

A representative sample can do the trick

Terms

Population = Entire group from which to collect data (e.g. county population; stores in a certain area, all city parks, etc.)

Census = all members of the population

Sample = a portion of the population

More terms

Sampling unit = the objects, people, timeslots, etc. that are being sampled (stores, residents, casino patrons, etc)

Sampling frame = How you derive your sample (phone book, street corner traffic…)

Sampling Methods

Simple Random Stratified Random Cluster Convenience Purposive

Simple Random

Each member of the population has the same chance of being in the sample

Stratified Random

A sample of equal size is drawn fromdifferent sub groups of the population

Use example:TRL: different neighborhoods that have different foreseeable consequences if TRL has passed – you want equal representation of all groups

Cluster

B

C

A

A = ruralB = urbanC = college town

Census ofAll members ofthe population in a group, e.g. all tobacco retailers

Purposive

Who or what you sample depends on the purpose of the use for the data

Examples KIIs generally POP: only those who smoke at the fair Observation: only parks with tot lots

Convenience Reason: limited time and capacity (give

reason)

Examples: Intercept public opinion surveys YTPS only when youth are available to do

stings Only housing residents who come to a

housing association meeting

Sample Size Sample size = The number of

population members you will use to collect data

Confidence level = The level of

certainty (usually set at 95%) that your sample represents the whole population

Confidence interval = The percentage of error you expect in your results

Population Confidence Confidence Sample

size level interval size 150 95% 10 59 150 95% 6 96 150 95% 4 120

Question

What is the minimum POS sample size to make statistically valid decisions for a population of 1.8 million?

Go to http://surveysystem.com Research Aids Sample Size Calculator

Sample size calculator

Determine Sample Size Confidence Level: _x_ 95% __99%Confidence Interval: 5 (2)Population:    1,800,000  

Sample size needed: 384 (2398)

What it means for analysisExampleSurvey Question:

Please say if you agree or disagree with the following statement:

“Smoking can shorten a person’s life.”__ Agree __ Disagree

Let’s say 75% of those asked said “agree.”

Analysis at confidence interval 5: We can say with 95% certainty that between 70 and 80 % (75 plus/minus 5) of the population in the county agree that smoking can shorten a person’s life.

Analysis at confidence interval 2: We can say with 95% certainty that between 73 and 77 % (75 plus/minus 2) of the population in the county agree that smoking can shorten a person’s life.

How to reportIn evaluation plan

A public opinion survey will be conducted with a sample size of 384 county residents randomly selected from the phone book (or “through a convenience sample”), for a 95% confidence level and a + 5 % confidence interval for a total of 1.8 million county residents.

(Note 1: you have to start out with a bigger sample size since a large percentage will decline to participate)

(Note 2: if you analyze sub-groups of your survey, let’s say an ethnic groups’ collective responses, your sample size is much smaller)

Determining statistical power afterwards

Resource:

http://www.greatbrook.com/survey_statistical_confidence.htm

Requirements

Q: Do I need to do sample size calculations?

Reviewers like to see it but will not require it. An estimated sample size is acceptable if it is at an “acceptable” rate, with acceptable referring to reasonableness with regards to available resources (in this case “200” will usually satisfy reviewers).

Other sample sizes (ss)

KII policy makers: ............(5-6) POS w/MUH residents: ……use calculator/ estimate ss Observation people at event:……………………………estimate total event

participants/ calculate or estimate ss

Tobacco litter …………………See “Sampling Plan” in Tips and Tools # 8,

Observation

Representativeness

Sampling method determines representativeness

Less likely inConvenience sample (e.g. intercept survey)

More likely inRandom samples

Question

How do you deal with small sample sizes in a survey?

If the survey is not yet completed – add to the sample

If survey is completed – report results, add limitations explanation; acceptance of results is still quite possible

Planning versus Analysis

What to focus on duringPlanning

“population” size, sample size, sampling method,

AnalysisRepresentativeness, limitations

Resources

TCEC website: Tips and Tools # 8: Observation Tips and Tools # 9: Sampling

Decisions OTIS project plans and reportsIndividual assistance: Call TCEC at 530-752-9951 E-mail at tobaccoeval@ucdavis.edu

Announcements

New Evaluation Associate TCEC will be on facebook A webinar on Cost-effective TC

Evaluation this Thursday at 10 (same phone number and website)