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ASRB Agricultural Research Service Bioassay, Survival & Epidemiological Statistics: practice questions

ASRB Agricultural Research Service (ARS) - Agricultural Statistics scientist · India · Agricultural Scientists Recruitment Board (ICAR)

Original practice questions written to the published syllabus · not past papers
10worked here, free
71written to this exam in this subject
11syllabus units

What this subject covers

Dose and response, time to event and disease in populations: direct and indirect assays, probit and logit analysis of LD50 and LC50, parallel-line and slope-ratio assays, Kaplan-Meier and Cox models, and the statistical side of plant and animal epidemiology. Introductory veterinary epidemiology lives in the Veterinary track; this is the statistics behind it.

In the exams: Light in AICE-JRF Ag Stat; ICAR MSc/PhD elective; plant-protection and veterinary research (applied); UPSC ISS (light)

Where the 71 questions fall

How our questions for ASRB Agricultural Research Service spread across the units of Bioassay, Survival & Epidemiological Statistics. Difficulty: 13 easy, 46 medium, 12 hard.

Diagnostic-test evaluation: sensitivity, specificity and ROC curves9
Disease progress curves and the area under the disease progress curve9
Measures of disease frequency and association: risk ratio and odds ratio8
Study designs: cohort, case-control and cross-sectional8
Direct and indirect bioassays7
Kaplan-Meier estimator and the log-rank test6
Quantal response: probit and logit analysis, LD50 and LC505
Survival data: censoring and truncation5
Cox proportional hazards model5
Parametric survival models: exponential and Weibull5
Parallel-line and slope-ratio assays; Fieller's theorem4

10 practice questions, worked

Choose an answer, then open the working. Each explanation says why the right answer is right and why the others are not.

1 · Quantal response: probit and logit analysis, LD50 and LC50 · medium

A field aphid population has LC50 = 120 mg/L and a susceptible strain 8 mg/L. What is the resistance ratio?

  1. 15
  2. 128
  3. 112
  4. 0.067
Answer and working

A. 15

Resistance ratio = LC50(field)/LC50(susceptible) = 120/8 = 15. 112 is a difference, not a ratio.

Further reading: Finney, Statistical Method in Biological Assay; Klein and Moeschberger, Survival Analysis; Dohoo, Martin and Stryhn, Veterinary Epidemiologic Research; Madden, Hughes and van den Bosch, The Study of Plant Disease Epidemics

2 · Quantal response: probit and logit analysis, LD50 and LC50 · medium

Why are doses giving 0% or 100% mortality of little use in estimating a probit line?

  1. They cause the LD50 to equal zero
  2. Little information on the line
  3. They make the probit undefined in software
  4. They are always excluded by regulation
Answer and working

B. Little information on the line

Binomial information np(1 − p) and the probit weights vanish as p approaches 0 or 1, so such groups barely constrain the fit; design should target 10-90% responses.

Further reading: Finney, Statistical Method in Biological Assay; Klein and Moeschberger, Survival Analysis; Dohoo, Martin and Stryhn, Veterinary Epidemiologic Research; Madden, Hughes and van den Bosch, The Study of Plant Disease Epidemics

3 · Parametric survival models: exponential and Weibull · medium

An exponential survival model has hazard 0.02 per day. What is the median survival time?

  1. 25.0 days
  2. 0.0139 days
  3. 50.0 days
  4. 34.7 days
Answer and working

D. 34.7 days

Median = (log 2)/λ = 0.693/0.02 = 34.7 days; 50 days is the mean.

Further reading: Finney, Statistical Method in Biological Assay; Klein and Moeschberger, Survival Analysis; Dohoo, Martin and Stryhn, Veterinary Epidemiologic Research; Madden, Hughes and van den Bosch, The Study of Plant Disease Epidemics

4 · Measures of disease frequency and association: risk ratio and odds ratio · easy

Thirty of 200 animals tested on one day are positive. What is this measure?

  1. Point prevalence, 0.15
  2. Incidence rate, 0.15 per day
  3. Incidence risk, 0.15
  4. Attack rate, 0.30
Answer and working

A. Point prevalence, 0.15

A proportion diseased at one time is a prevalence; incidence needs new cases over a period.

Further reading: Finney, Statistical Method in Biological Assay; Klein and Moeschberger, Survival Analysis; Dohoo, Martin and Stryhn, Veterinary Epidemiologic Research; Madden, Hughes and van den Bosch, The Study of Plant Disease Epidemics

5 · Measures of disease frequency and association: risk ratio and odds ratio · easy

Twelve new cases occur in 4,800 animal-days at risk. What is the incidence rate?

  1. 0.025 per animal-day
  2. 12 per 4,800 animals
  3. 0.25 per animal-day
  4. 0.0025 per animal-day
Answer and working

D. 0.0025 per animal-day

Rate = cases/animal-time = 12/4,800 = 0.0025 per animal-day (about 0.91 per animal-year).

Further reading: Finney, Statistical Method in Biological Assay; Klein and Moeschberger, Survival Analysis; Dohoo, Martin and Stryhn, Veterinary Epidemiologic Research; Madden, Hughes and van den Bosch, The Study of Plant Disease Epidemics

6 · Measures of disease frequency and association: risk ratio and odds ratio · medium

In plant pathology, what does disease 'incidence' usually measure?

  1. The rate of new infections per day
  2. The proportion of plant units showing disease
  3. The proportion of tissue area affected
  4. The yield loss per infected plant
Answer and working

B. The proportion of plant units showing disease

Plant-pathology incidence counts diseased units (plants, leaves) — a prevalence in the epidemiological sense; severity measures tissue affected.

Further reading: Finney, Statistical Method in Biological Assay; Klein and Moeschberger, Survival Analysis; Dohoo, Martin and Stryhn, Veterinary Epidemiologic Research; Madden, Hughes and van den Bosch, The Study of Plant Disease Epidemics

7 · Study designs: cohort, case-control and cross-sectional · easy

Which design selects subjects by disease status and compares their past exposures?

  1. Randomised field trial
  2. Cross-sectional
  3. Case-control
  4. Cohort
Answer and working

C. Case-control

Cases and controls are sampled by outcome; cohort studies sample by exposure and follow forward.

Further reading: Finney, Statistical Method in Biological Assay; Klein and Moeschberger, Survival Analysis; Dohoo, Martin and Stryhn, Veterinary Epidemiologic Research; Madden, Hughes and van den Bosch, The Study of Plant Disease Epidemics

8 · Diagnostic-test evaluation: sensitivity, specificity and ROC curves · medium

Positives from a screening test are confirmed by a second test before culling. Which property increases?

  1. Prevalence
  2. Sensitivity
  3. Apparent prevalence
  4. Specificity
Answer and working

D. Specificity

Serial interpretation (positive only if both positive) raises specificity and lowers sensitivity.

Further reading: Finney, Statistical Method in Biological Assay; Klein and Moeschberger, Survival Analysis; Dohoo, Martin and Stryhn, Veterinary Epidemiologic Research; Madden, Hughes and van den Bosch, The Study of Plant Disease Epidemics

9 · Disease progress curves and the area under the disease progress curve · medium

An AUDPC of 315 %-days covers 14 days. What is the standardised AUDPC?

  1. 22.5%
  2. 45%
  3. 4.4%
  4. 315%
Answer and working

A. 22.5%

Standardised AUDPC = AUDPC/duration = 315/14 = 22.5%, the average severity over the period.

Further reading: Finney, Statistical Method in Biological Assay; Klein and Moeschberger, Survival Analysis; Dohoo, Martin and Stryhn, Veterinary Epidemiologic Research; Madden, Hughes and van den Bosch, The Study of Plant Disease Epidemics

10 · Disease progress curves and the area under the disease progress curve · medium

Which model suits a monocyclic soil-borne disease whose inoculum does not increase in the season?

  1. Gompertz
  2. Logistic
  3. Monomolecular
  4. Exponential
Answer and working

C. Monomolecular

Monomolecular growth dy/dt = r(1 − y) reflects infection from a fixed inoculum; logistic growth suits polycyclic spread.

Further reading: Finney, Statistical Method in Biological Assay; Klein and Moeschberger, Survival Analysis; Dohoo, Martin and Stryhn, Veterinary Epidemiologic Research; Madden, Hughes and van den Bosch, The Study of Plant Disease Epidemics

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Read the subject

Bioassay, Survival & Epidemiological StatisticsThe full study manual, free ASRB Agricultural Research ServiceFormat, sections and how it has changed

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