Sampling

CLUSTER SAMPLING is effective under what CONDITIONS?

1) A good SAMPLING FRAME is NOT AVAILABLE or costly, while a frame listing clusters is easily obtained. 2) The cost of obtaining observations increases as the distance separating the elements increases.

DIFFERENCES between STRATA and CLUSTERS?

1) ALL STARTA are represented in the SAMPLE; but only a SUBSET of CLUSTERS are in the SAMPLE. 2) With STRATIFIED SAMPLING, the BEST survey results occur when elements within strata are INTERNALLY HOMOGENEOUS. 3) With CLUSTER SAMPLING, the BEST results occur when elements within clusters are INTERNALLY HETEROGENEOUS

CLUSTER SAMPLING PROCESS?

1) DIVIDE the population into groups (clusters) 2) RANDOMLY select some of the CLUSTERS, and collect DATA from all members of the selected clusters

How is QUOTA SAMPLING conducted

1) IDENTIFY stratums and their proportions as they are represented in the population 2)Then convenience or judgment sampling is used to select the required number of subjects from each stratum

What is the PROCEDURE used for SNOWBALL SAMPLING?

1) INITIAL GROUP of respondents is SELECTED (usually by convenience) 2) After data collection, these respondents are asked to IDENTIFY OTHERS who belong to the target population of interest

sampling procress

1) define the POPULATION 2) detemine the SAMPLING FRAME 3) select TECHNIQUES 4) determine SAMPLE SIZE 5) execute the sampling PROCESS

Why SAMPLE?

1) lack of money 2) Large population of interest

What are the benefits of STARTIFIED SAMPLING?

1) reduces sampling error 2) results in a more REPRESENTATIVE sample

ADVANTAGES to CLUSTER SAMPLING?

1)Cuts down on the COST of preparing a sampling frame. 2)Can reduce travel and other administrative COSTS.

2 key rules of STRATIFIED SAMPLING?

1)Elements within a stratum should be as homogeneous as possible, but the elements in different strata should be as heterogeneous as possible. 2)The stratification variables should also be closely related to the characteristic of interest.

PROCEDURE for STARTIFIED SAMPLING?

1)IDENTIFIY RELEVANT STRATUMS and their actual representation in the population. 2)RANDOM SAMPLING is then used to select subjects from each stratum.

JUDGEMENT SAMPLING?

A commonly used NONPROBABILITY METHOD. Extension of CONVENIENCE SAMPLING ***Researcher should try to assess the representativeness of the chosen sample.

What is a SAMPLING FRAME?

Has the property that we can identify every single element and include any element in our sample. REPRESENTATIVE of the population

When is CONVENIENCE SAMPLING used?

In EXPLORATORY RESEARCH where the researcher is interested in getting an inexpensive APPROXIMATION.

PROPORTIONATE STRATIFIED SAMPLING?

Size of the sample drawn from each stratum is PROPORTIONATE to the relative size of that STRATUM in the TOTAL PROPULATION. (broad-applicable to an entire population)

DISPROPORTIONATE STATIFIED SAMPLING

Size of the sample from each stratum is DISPROPORTIONATE to the relative size of that STRATUM in the total population. (specific to a strata)

"Errors" in Sampling

The DEVIATION between an ESTIMATE from an ideal sample and the TRUE POPULATION VALUE is the sampling error.

QUOTA SAMPLING?

The NONPROBABILITY equivalent of STARTIFIED SAMPLING.

what determines SAMPLE SIZE?

The more HETEROGENEOUS a population is, the LARGER the sample needs to be.

RANDOM sampling

The purest form of PROBABILITY sampling. Each member of the population has an EQUAL, NON-ZERO and KNOWN chance of being selected.

CONVENIENCE SAMPLING

The sample is selected because they are convenient. It is a NONPROBABILITY method

what is the biggest challenge of RANDOM sampling

With large populations, it is 'difficult' to identify every member of the population (You can use software to generate random numbers)

SIMILARITY between STRATA and CLUSTERS?

HETEROGENEOUS: strata and clusters are both NON-OVERLAPPING SUBSETS of the population

SNOWBALL SAMPLING?

A special NONPROBABILITY method used when the desired sample characteristic is rare. Relies on REFERRALS from initial subjects to generate additional subjects.

what is a STRATUM

A subset of the population that share at least one common characteristic Ex. males and females, nursing program (BSN vs MSN).

What is your POPULATION OF INTEREST?

All PEOPLE or items (elements) with the CHARACTERISTICS one wishes to understand and make inferences about. defined in connection with the objectives of the study ***To whom do you want to generalize your results?***

why are NONPROBABILITY SAMPLES not generalizable regardless of size?

Beccause they are NONRANDOM?

What are some NONPROBABILITY sampling methods

CONVENIENCE sampling JUDGEMENT sampling QUOTA sampling SNOWBALL sampling

PROBABILITY samples?

Each member of the population has a known NON-ZERO PROBABILITY of being selected (RANDOM)

NONPROBABILITY samples?

Members are selected from the population in some NONRANDOM manner (ZERO PROBABLITY)

Does the TARGET population need to be the same as the sample?

No, but there needs to be an overlap between these two groups. ex. we study records from people born in 2008 in order to make predictions about people born in 2009.

CENSUS SAMPLE?

Occurs if the entire POPULATION is SMALL or it is reasonable to include the entire population.

STRATIFIED SAMPLING?

PROBABILITY METHOD Population is partitioned into SUBPOPULATIONS, or STRATA.

CLUSTER SAMPLING

PROBABILITY SAMPLE in which each sampling unit is a collection of elements.

MULTISTAGE SAMPLING

Process of taking random samples of PRECEDING random samples.

What are some PROBABILITY sampling methods?

RANDOM sampling SYSTEMATIC sampling STRATIFIED sampling CLUSTER Sampling Other - Combination (Multi-stage)

DISTADVANTAGE to CLUSTER SAMPLING?

Sampling ERROR is higher than for a simple random sample of same size

Factors that influence SAMPLE REPRESENTATIVENESS?

Sampling procedure Sample size Participation (response rate)

SYSTEMATIC SAMPLING?

Select a RANDOM STARTING POINT and then select every Kth subject in the population could be as good as the random sampling method. K-X-X-K-X-X-K-X-X

When is STRATIFIED SAMPLING used?

Stratified sampling is often used when one or more of the stratums in the population have a LOW INCIDENCE relative to the other stratums

FACTORS in Determining SAMPLE SIZE?

Study design The nature of the analysis The number of variables Expected variability in the data Expected effect size of the intervention Incidence rates of outcome of interest Completion rates (missing data?) Attrition rates (for longitudinal studies) Resource constraints

How is the sample selected in JUDGEMENT SAMPLING?

based upon JUDGEMENT

why is it called a CENSUS SAMPLE

data is gathered on EVERY MEMEBER of the population

what is the GOAL of sampling?

finding a REPRESENTATIVE (SUBSET) of the population of interest

Who uses CLUSTER SAMPLING?

government & private research organizations Examples of clusters: Schools - students Enumeration areas - families, individuals

What must the sample reflect

the CHARACTERISTICS of the population from which it is drawn

When is MULTISTAGE SAMPLING used?

when a COMPLETE LIST of all members of the population does NOT EXIST, or is INAPPROPRIATE.