animal disease surveys

5/25/2018 Animal Disease Surveys

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An imal disease

surveys


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Veterinary surveys

Information may be obtained from:

Censuses

Sample surveys


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To gain information on:

Counts (e.g. prevalence)

Measurements (e.g. milk yield)


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Surveys of widespread relevance:

National picture of disease(e.g. lameness, mastitis)

National and regional control and

eradication campaigns(e.g. rinderpest)

Local (farm) eradication and controlcampaigns (e.g. enzootic abortion)


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Bias

Systematic(as opposed torandom)

departures from true values


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Veterinary surveys:

Definitions

Target population

Study population

Elementary units

Stratum

Sampling frame

Sampling unit(may, or may not, be elementary units)

Sampling fraction


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Types of sampling

1.Non-probability sampling:

2. Probability sampling:

- choice of investigator

- a deliberate unbiasedprocess

- each sampling unit has an equalprobability of selection

- basis of random sampling


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Non-probability sampling methods

Convenience sampling

Purposive selection


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Probability sampling methods

Simple random sampling

Systematic sampling

Stratified sampling

Cluster sampling


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Random sampling

Methods:1. Tables

2. Pocket calculators

3. Software


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Region Number of

cows

Number sampled

Devon and Cornwall 302 647 302 647 x 0.05 = 15 132SW England other than

Devon and Cornwall

469 486 469 486 x 0.05 = 23 474

S England 271 225 271 225 x 0.05 = 13 561

E England 119 835 119 835 x 0.05 = 5 992

East Midlands 189 817 189 817 x 0.05 = 9 491

West Midlands 462 826 462 826 x 0.05 = 23 141

Wales 342 346 342 346 x 0.05 = 17 117

Yorkshire/Lancashire 255 626 255 626 x 0.05 = 12 781

N England 273 838 273 838 x 0.05 = 13 692

Scotland 260 366 260 366 x 0.05 = 13 018

Totals 2 948 012 147 399


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50300 1020 40 km

A

B

C

PUNAKHA

WANGDI

BUMTHANG

LHUNTSHI

THIMPHU

PARDHA

PHODRANG TONSA

SAMCHI

CHHUKHA

DAGA

CHIRANGLEYLEGPRUG

SHEMGANG

MONGAR

TASHIGANG

SAMDRUP

JONGKHAR

PEMA-

GATSEL

Thimphu

A

B

C

PUNAKHA

WANGDI

BUMTHANG

LHUNTSHI

THIMPHU

PARDHA

PHODRANG TONSA

SAMCHI

CHHUKHA

DAGA

CHIRANGLEYLEGPRUG

SHEMGANG

MONGAR

TASHIGANG

SAMDRUP

JONGKHAR

PEMA-

GATSEL

Thimphu


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Cluster sampling

Used when:

1. Sampling frame is incomplete

2. Random sampling from all clusters is

impracticable

May be:

1. One-stage: sampleallanimals in

selected clusters2. Two-stage: sample someanimals in

selected clusters

{3. Multistage}


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Sample size determination

Estimation of disease prevalence

Detecting the presence of disease


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Sample size determination

Not a matter of guess-workor convenience

For proportions, depends on:

Expected proportion (P)

Precision of sample estimate (d)

Degree of confidence in estimate

(usually 95%)

Population size


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How many animals?

Simple random sampling

n=1.962 Pexp(1-Pexp)

d2

Where:

n = required sample size

Pexp = expected prevalence

d = desired absolute precision


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n = 1.962 0.30 (1-0.30)

0.052

Pexp = 0.30

d = 0.05

Example:

= 3.84 x 0.21

0.0025

= 323


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nadj = (Nx n) / (N+ n)

where N= population size

Finite correction

N = 900Example

nadj = (900 x 323) / (900 + 323)

= 238

S l i i d t tt i d i d fid


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Sample size required to attain desired confidenceinterval around expected percentage of 5%

Percenta e

6000

4000

2000

1000800

600

400

200

10080

60

40

20

0 5 10 15 20 25 30 35 40 45

95%

95%

99%

6000

4000

2000

1000800

600

400

200

10080

60

40

20

99%

Re

quiredsamplesize


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If an idea of estimated prevalence

is not available ( a common event!)

What do we do??

Assume 0.5 (50%)


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Detecting the presence of disease

(Certifying absence )

n= {1(1-p1)1/d} {N-d/2} + 1,

Where:

N = population size

d = number of affected animalsin the population

n = required sample size

p1 = probability of finding atleast one case in the sample


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Example: (Pan-African Rinderpest Campaign)

N = 200d = 10 (5% of 200)

N = {1- (1-0.95)1/10} {20010/2} + 1

p1 = 0.95

Assume at least 5% seroconvert in infected herds Set p1at 0.95

Apply to herd size 200

= {1- 0.051/10} x 195 + 1= {1- 0.74113} x 195 + 1

= 0.2589 x 195 + 1

= 50 + 1

= 51


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FMD serosurveillance

EU council directive 85/511/EC Probability 0.95

Flock prevalence: 2%

Within-flock prevalence: 5%


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Sampling frames

Dumfries & Galloway (+ Wigton):752 premises

Duns: 176 premises

Jedburgh: 129 premises


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Samples: premises (animals)

Dumfries & Galloway (+ Wigton):138 (18,040)

Duns: 102 (11,187)

Jedburgh: 91 (10,614)


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Calculation of confidence intervals:

Simple proportion

(e.g., simple random sampling)

1. Exact method (binomial)

2. Approximate methods:

Normal approximation

Poisson approximation(for rare diseases)

C fid i t l f l


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P* = Sample estimate

n = Number in sample

Thus:P* = 0.40

E.g., in a sample 100 animals, taken from a population of

10,000, 40 are diseased.

Normal approximation:for a 95% interval

Where:

95% c.i. = 0.401.96 (0.40 x 0.60 / 100)

Confidence intervals for prevalence:simple random samples

P*- 1.96P* (1-P*)

n, P*+ 1.96

P* (1-P*)n

to 0.40 + 1.96 (0.40 x 0.60 / 100)= (0.400.096), (0.40 + 0.096)= 0.304, 0.496

30.4% - 49.6%


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Finite correction:

If the sampling fraction, f, is large (say,

>10%):

The numerator, P* (1-P*), should be

multiplied by (1-f)

E.g., if a sample of 100 animals is drawn

from a population of total size 500, thenf= 0.20, and (1-f ) = 0.80


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Multipliers used in construction of

common confidence intervals, based

on the Normal distribution

Confidence

interval

80% 90% 95% 99% 99.9%

Multiplier 1.282 1.645 1.960 2.576 3.291

Confidence intervals for


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If P* = estimated prevalence

andn = sample size

1. P 0.05

Formula should only be used when:

Restriction on asymptotic method:

and

Confidence intervals forsimple random samples:

2. nPand n(1-P) 5

If these conditions do not apply, use the exactbinomial method:

1. from tables

2. with appropriate software, e.g., WINPEPI


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Calculation of confidence intervals

Systematic sampling

If no periodicity:use simple proportion formula

Stratified samplingFor proportional allocation:

use simple proportion formula

Cluster sampling complex!!Formula for simple proportion will produce

too narrowa confidence interval

(i t i )

animal disease surveys

Documents

n england

sample size determinationnot

random samplingmethods

required sample sizepexp

simple random samplingn

sw england

s england

e england