Sampling of Animal Sampling of Animal PopulationsPopulations

Learning Objectives:Learning Objectives: define & differentiate samplingdefine & differentiate sampling advantages/disadvantages advantages/disadvantages

sampling methodsampling method select sampling strategyselect sampling strategy

Data SourcesData Sources

For epidemiological For epidemiological analyses, must analyses, must be:be:

CompletenessCompleteness

ValidityValidity

RepresentativeneRepresentativenessss

Accuracy, Refinement, Precision, Accuracy, Refinement, Precision, Reliability and Validity Reliability and Validity (Thrusfield, 1986)(Thrusfield, 1986)

Accuracy: investigation or measurement Accuracy: investigation or measurement conforms to the truthconforms to the truth

Refinement: eq, 13 kg and 13,781 kg, both Refinement: eq, 13 kg and 13,781 kg, both represent accurate, but the second is more represent accurate, but the second is more refined than the first. Another eq., otitis refined than the first. Another eq., otitis externa, otitis externa by bacteria externa, otitis externa by bacteria

Precision: as a synonym of refinement and Precision: as a synonym of refinement and to indicate the concistency of a series to indicate the concistency of a series measurements (repeated sampling)measurements (repeated sampling)

Accuracy, Refinement……….Accuracy, Refinement……….

Reliability (= reproducibility): Reliability (= reproducibility): produces similar results when its produces similar results when its repeatedrepeated

Validity: measure what its supposed Validity: measure what its supposed to measure, its long term to measure, its long term characteristic of tehniquecharacteristic of tehnique

Data can be collected as Data can be collected as part ofpart of

Routine data collection: laboratory Routine data collection: laboratory submissions, disease surveillance submissions, disease surveillance programmes, industry/farm data programmes, industry/farm data recording systemrecording system

Structured data collection: regular Structured data collection: regular monitoring of disease/productionmonitoring of disease/production

Epidemiological studiesEpidemiological studies

Data Collection ProcessData Collection Process

Whole population interest Whole population interest (=cencus)(=cencus)

It can be restricted to a It can be restricted to a samplesample : : obtained more quickly, less obtained more quickly, less expensive to collect, more expensive to collect, more accurate, more efficientaccurate, more efficient

SamplesSamples

Probability Samples : random Probability Samples : random selectionsselections

Non-Probability Samples: a Non-Probability Samples: a convenience sample, a convenience sample, a purposive or judgmental purposive or judgmental samplesample

Probability SamplingProbability Sampling

Simple Random Simple Random SamplingSampling

Systematic SamplingSystematic Sampling Stratified SamplingStratified Sampling Cluster SamplingCluster Sampling Multistage SamplingMultistage Sampling

Simple Random Sampling

Each individual has an equal probability of selection

An individual’s selection doesn’t depend on others being selected, homogenous population

Disadvantage : may result large variation, thereby requiring larger sample sizes

Eq: flipping a coin, using random number tables

Ex: catlle on farm

Systematic Random Sampling

The n sampling units are selected from the sampling frame at the regular intervals

The starting point in the first interval is selected on a formal random basis

A practical way to obtain a representative sample

It ensures that sampling units are distributed evenly over the entire population

Stratified Random Sampling

The sampling frame is divided into strata, then a simple random or systematic random sample is selected within each stratum

To be effective at reducing variation, for example: milk production in population of dairy cows of the Jersey and Holstein breeds. Genetic differences affecting milk volume between the two breeds.

Definite strata, but homogenous within it

Cluster Sampling

Sampling is applied at an aggregated level (=group) of individual units

Groups or clusters such litters, pens, herds, artificial groupings (geographic areas)

Can be selected by simple, systematic, or stratified random methods

Groups with similar characteristics, but heterogenous within groups

Multistage Sampling

Similar to cluster except that sampling takes place at all stages/at different hiererchical levels og aggregated units of interest

Subsampling within the primary units (litters, pens, herds). A sampling of secondary units (e.q., animals) would be selected

Often used as part of epidemiological studies Ex: cattle in region, to be sampled to

determine TBC or mastitis prevalence

Sampling to detect Sampling to detect diseasedisease

Finite populations:Finite populations:

n = [1-(1-n = [1-(1-ββ) ] [ ( N- d/2) + ½]) ] [ ( N- d/2) + ½]

Infinite populations (> 1000)Infinite populations (> 1000)

n = [ log (1-n = [ log (1-ββ)] / [ log ( 1- d/N) ])] / [ log ( 1- d/N) ]