EPI study design: intervention studies

I n t e r v e n t i o n s t u d i e s :

C o n t r o l l e d t r i a l s / e x p e r i m e n t a l s t u d i e s

Differ from observational

studies in that the

researcher has control

over the allocation of the

study subjects to the

groups being compared.

Other characteristics

• Clinical trial or Field trial

• Randomised or non-

randomised

D o i n t e r v e n t i o n s t u d i e s f u l f i l

B r a d f o r d h i l l c r i t e r i a f o r c a u s a l i t y ?

ntervention studies can meet the criteria very well (i.e., demonstrate cause-

and-effect relationships) – if rigorous

randomised clinica trial/field trial

A randomised clinical trial (RCT) or field trial is the gold standard for

demonstrating causality.

Several classifications are possible based on outcome and host:

• Therapeutic trials are conducted among individuals with a particular disease to assess the

effectiveness of an agent or procedure to diminish symptoms, prevent recurrence, or reduce

mortality from the disease.

• Preventative trials are conducted to evaluate whether an agent or procedure reduces the risk of

developing a particular disease among individuals free from that disease at the beginning of the

trial, for example, vaccine trials. Preventative trials may be conducted among individuals or among

entire communities.

—Vaccine efficacy studies.

• Individual vs community trial

phases of clinical trials

Typical approach in evaluating new human drugs, vaccines etc

controlled trial/intervention study - when do we conduct intervention studies?

Intervention

• E.g., to test efficacy against no treatment; to compare to an existing

treatment & demonstrate equivalence; to compare to an existing product

and demonstrate superior performance.

Outcome measure

• Clinical improvement, change in a clinical test parameter, survival, time

to recovery etc.

I n t e r v e n t i o n S t u d i e s / C o n t r o l l e d

T r i a l s – K e y C o n s i d e r a t i o n s

• Background information

• Study objective

• Outcome

• Study population

— Inclusion & exclusion criteria

• Sample size

• Enrolment of animals

• Intervention

• Bias control

• Data collection & analysis

• Report of results

C o n t r o l l e d T r i a l / I n t e r v e n t i o n S t u d y

Specify intervention

• Intervention/treatment group: Therapeutic (medication) or Preventative

(vaccines/management factors).

• Control group: Other/placebo.

Animals grouped by researcher according to the intervention into

intervention/treatment groups or control groups

• Randomly allocated to minimise bias – groups are comparable.

• Non-randomly allocated - groups not comparable.

Follow-up and compliance

Masking/blinding: to know or not to know

• Everyone knows (e.g. change in management

strategy).

• Single blind: Observer is blind to category of

intervention.

• Double blind: Observer and researcher

(analysing the data) blind to category of

intervention.

R a n d o m i s e d F i e l d T r i a l : E x a m p l e 1

Objective: To assess the efficacy of prophylactic hoof health examination and

trimming during mid-lactation at reducing the incidence of lameness during

late lactation in Holstein cow

R a n d o m i s e d F i e l d T r i a l : E x a m p l e 1

I n t e r v e n t i o n S t u d i e s : L i m i t a t i o n s

• Intervention studies may be expensive – time and resource

intensive.

• Results of clinical trials (set in ‘lab’ or ‘controlled’ environment)

may not be repeatable in ‘real’ world settings.

• Informed consent & ethics issues.

• Sample size must be adequate to detect an important difference

between groups – this applies to other types of studies

Intervention study: Example 2

You have been provided with a scientific paper entitled:

Efficacy of Two Modified Nonantibiotic Formulations

(Victory) for Treatment of Papillomatous Digital Dermatitis in

Dairy Cows.

• The paper describes an intervention study in cattle.

• Students are encouraged to read the full article

ahead of the class session. The PDF article is

provided on SurreyLearn.

• Activities: Identify and explain the study population

used in the article, sampling approach, intervention,

allocation of animals to groups, outcomes of interest,

analysis – parameters of interest compared between

groups, and the study limitations.

Example: Efficacy of Two Modified Nonantibiotic Formulations (Victory)

for Treatment of Papillomatous Digital Dermatitis in Dairy Cows

➢Study objective

➢Study population

➢Sampling approach

➢Intervention

➢Allocation of animals to groups

➢Outcomes of interest

➢Analysis – parameters of interest compared between groups

➢Study limitations

Example: Efficacy of Two Modified Nonantibiotic Formulations (Victory)

for Treatment of Papillomatous Digital Dermatitis in Dairy Cows

➢ Study objective

• To compare the efficacy of the original and two modified formulations of Victory and oxytetracycline among dairy cows affected with Papillomatous Digital Dermatitis (PDD).

➢Study population

• Dairy cattle herd with approximately 3200 lactating cows.

• Herd with a history of lameness associated with PDD.

➢Sampling approach

• Cows with PDD lesions enrolled

Example: Efficacy of Two Modified Nonantibiotic Formulations for Treatment of Papillomatous Digital Dermatitis in Dairy Cow

C o n t r o l l e d T r i a l / I n t e r v e n t i o n S t u d y

S u m m a r y : C o n t r o l l e d t r i a l s o r I n t e r v e n t i o n s t u d i e s

• Researcher defines intervention/treatment group and a control group

(i.e. experiment is planned).

• Random allocation of sampling units to groups.

• Comparison of intervention/treatment group and a control group.

— Study groups are comparable at start.

• Groups followed up to establish & measure outcome.

• Can provide reliable evidence for causality – if study design is rigorous.