Study Design

1
Study Design

• Mark Stevenson
• EpiCentre, IVABS, Massey University, Palmerston
  North
• M.Stevenson_at_massey.ac.nz

2
Roadmap

• Background
• Descriptive studies
• Analytical studies
• Experimental studies

3
Background

• How do we learn veterinary medicine?
• vet school lectures, practical sessions
• conferences
• text books
• journal articles
• The mix of diseases (and their relative
  importance) that youll encounter in practice is
  likely to be quite different to what you learn in
  vet school

4
Background

• As a practitioner, its important that you
  recognise and hopefully devise ways to
  understand disease syndromes that are unique to
  your area of practice
• This ability will make you a good vet

5
From Joe Practitioner j.p.practitioner_at_netspeed.
net.nzSent Friday, 17 March 2006 1206
p.m.To Mark StevensonSubject PEMarkI am
very interested in submitting an application for
funding looking at the causes of PE
(polioencephalomalacia) in calves - I am sure
there is more to the picture than we think. It is
now our most common disease in calves - and it
doesn't appear to be linked to the test book
explanation of change in diet from fibrous to
lush (or sulphur content).I am going to contact
4 - 5 other practices in the southern South
Island, to see if they are experiencing the same
increase in PE. I am a little unsure what data I
should be trying to find out from them. I will
need to know - the number of cases they deal with
per year, the number of affected animals within
the herd/mob affected - incidence? Number of
deaths - mortality. Guesstimate on possible
production loss. What they were grazing at the
time - changes in diet associated with
outbreak? Would you be able to give me a hand to
ask/phrase the 5 - 6 questions that I need to ask
veterinary collegeges - sorta information that
they would be easily able to give me, so I can
get a general picture of the significance of PE
in other parts of the South Island. I see there
was an article on PE in this months'
Vetscript.Thanks for your time.Regards,
6
Background

• Practitioners should be prepared to design and
  conduct field studies that allow them to
• quantify the frequency of disease in populations
• identify risk factors for disease
• This lecture outlines some approaches (? study
  designs) that can be used

7
Roadmap

• Background
• Descriptive studies
• Analytical studies
• Experimental studies

8
Descriptive studies

• Often the earliest studies done on a new disease
  in order to characterise it, quantify its
  frequency, and determine how it varies in
  relation to
• individual
• place
• time
• Undertaken without a specific hypothesis

9
Descriptive studies

• Types
• 1. Case report describes some newsworthy
  clinical occurrence, such as an unusual
  combination of clinical signs, experience with a
  novel treatment, or a sequence of events
• 2. Case series whereas a case report shows that
  something can happen once, a case series shows
  that it can happen repeatedly
• 3. Descriptive studies quantify the burden of
  disease in a population. Provide a source of
  hypotheses that can later be tested

10
Case report
11
Case series
12
Descriptive study
13
Roadmap

• Background
• Descriptive studies
• Analytical studies
• Experimental studies

14
Analytical studies

• Analytical studies are undertaken to test a
  hypothesis
• In epidemiology the hypothesis typically involves
  working out whether a certain exposure is
  associated with (or causes) a certain outcome

15
Analytical studies

• Exposures include traits, behaviours, and (or)
  environmental factors that might be associated
  with or cause disease
• synonyms potential risk factor, putative cause,
  independent variable, predictor
• Outcome generally refers to the occurrence of
  disease
• synonyms effect, end-point, dependent variable

16
Analytical studies

• Types
• 1. Ecological studies unit of analysis is a
  group of individuals (such as counties, states,
  cities, or census tracts) and summary measures of
  exposure and outcome are compared
• 2. Cross-sectional studies a sample of
  individuals from a population is taken at a point
  in time and examined for the presence of disease
  and their status with regard to the presence or
  absence of specified risk factors

17
Analytical studies

• Types
• 3. Cohort studies involves comparing disease
  incidence over time between groups (cohorts) that
  are found to differ on their exposure to a factor
  of interest
• 4. Case-control studies involves comparing the
  frequency of past exposure between cases and
  controls (individuals chosen to reflect the
  frequency of exposure in the underlying
  population at risk)
• 5. Hybrid designs see notes

18
1. Ecological studies

• Unit of analysis is a group of individuals (such
  as counties, states, cities, or census tracts)
• Summary measures of exposure and summary measures
  of outcome are compared

19
Ecological study comparison of cancer rates in
municipalities with and without chlorinated
drinking water
20
(No Transcript)
21
BSE SMR cattle born after July 1988
22
1. Ecological studies
23
2. Cross-sectional studies

• Random sample of individuals from a population is
  taken at a point in time (e.g. survey)
• Individuals are examined for the presence of
  disease and their status with regard to the
  presence or absence of specified risk factors

24
(No Transcript)
25
Cross sectional study
26
2. Cross-sectional studies
27
3. Cohort studies

• A cohort study involves comparing disease
  incidence over time between groups (cohorts) that
  are found to differ on their exposure to a factor
  of interest
• Cohort studies can be distinguished as either
  prospective or retrospective

28
(No Transcript)
29
Retrospective cohort study
30
3. Cohort studies
31
4. Case-control studies

• Comparing the frequency of past exposure between
  cases and a set of controls
• Controls are carefully chosen to reflect the
  frequency of exposure in the underlying
  population at risk

32
(No Transcript)
33
Case-control study
34
4. Case-control studies
35
Roadmap

• Background
• Descriptive studies
• Analytical studies
• Experimental studies

36
Experimental studies

• Experimental studies are also designed to test
  hypotheses between specific exposures and
  outcomes
• The major feature of experimental studies is that
  the investigator has direct control over the
  study conditions

37
Experimental studies

• Types
• 1. Randomised clinical trials
• 2. Community trials

38
1. Randomised clinical trials

• Closely resembles a laboratory experiment
• Objective is to test the possible effect of a
  therapeutic or preventive intervention
• The design's key feature is that a formal chance
  mechanism is used to assign participants to
  either the treatment or control group
• Subjects then followed over time to measure one
  or more outcomes, such as the occurrence of
  disease

39
(No Transcript)
40
Randomised clinical trial
41
(No Transcript)
42
1. Randomised clinical trials
43
2. Community trials

• Instead of randomly assigning individuals to
  treatment or control groups, community trials
  assign interventions to entire groups of
  individuals
• In the simplest situation one group (community)
  receives the treatment and another serves as a
  control

44
From Joe Practitioner j.p.practitioner_at_netspeed.
net.nzSent Friday, 17 March 2006 1206
p.m.To Mark StevensonSubject PEMarkI am
very interested in submitting an application for
funding looking at the causes of PE
(polioencephalomalacia) in calves - I am sure
there is more to the picture than we think. It is
now our most common disease in calves - and it
doesn't appear to be linked to the test book
explanation of change in diet from fibrous to
lush (or sulphur content).I am going to contact
4 - 5 other practices in the southern South
Island, to see if they are experiencing the same
increase in PE. I am a little unsure what data I
should be trying to find out from them. I will
need to know - the number of cases they deal with
per year, the number of affected animals within
the herd/mob affected - incidence? Number of
deaths - mortality. Guesstimate on possible
production loss. What they were grazing at the
time - changes in diet associated with
outbreak? Would you be able to give me a hand to
ask/phrase the 5 - 6 questions that I need to ask
veterinary collegeges - sorta information that
they would be easily able to give me, so I can
get a general picture of the significance of PE
in other parts of the South Island. I see there
was an article on PE in this months'
Vetscript.Thanks for your time.Regards,
45
Ideas for this practitioner?

• What is the problem case definition?
• What is the unit of interest?
• What type of study design would you use?

46
Roadmap

• Background
• Descriptive studies
• case reports, case series, descriptive
• Analytical studies
• ecological studies, cross-sectional studies,
  cohort studies, case-control studies
• Experimental studies
• randomised clinical trials, community trials