Statistics

Question 1

Cohort study

Answer 1

Observational and prospective. Two (or more) are selected according to their exposure to a particular agent (e.g. medicine, toxin) and followed up to see how many develop a disease or other outcome.

The usual outcome measure is the RELATIVE risk.

Examples include Framingham Heart Study.

A cohort study follows a group of people over time, looking at a specific outcome (e.g. death) and whether exposure to a given risk factor (e.g. smoking) contributes to that outcome.

Question 2

Case-control study

Answer 2

Observational and retrospective. Patients with a particular condition (cases) are identified and matched with controls. Data is then collected on past exposure to a possible causal agent for the condition.

The usual outcome measure is the odds ratio. we start with the disease status and then look back at the exposure. The odds ratio compares the odds of exposure in the cases to the odds of exposure in controls.

Inexpensive, produce quick results
Useful for studying rare conditions
Prone to confounding

Question 3

Cross-sectional survey

Answer 3

Provide a ‘snapshot’, sometimes called prevalence studies.

Provide weak evidence of cause and effect.

Question 4

Attributable proportion

Answer 4

Also known as the attributable risk percent this term refers to the proportion of a disease that could be eliminated in a population if its disease rate were reduced to that of the unexposed group.

In other words, it signifies how much of the disease burden in a population can be attributed to a specific exposure.

Question 5

Risk ratio

Answer 5

Represents the ratio of the probability of an event occurring in an exposed group compared to an unexposed group. It is used to determine whether there is a difference in risk between two groups.

Question 6

Attributable risk

Answer 6

Attributable risk refers to the difference in the rate of a condition between an exposed population and an unexposed population. It provides information about how many cases are due to exposure.

Question 7

Odds ratio

Answer 7

The odds ratio compares the odds of occurrence of an event in one group to that in another group.

An odds ratio (OR) is a measure of association between an exposure and an outcome. The OR represents the odds that an outcome will occur given a particular exposure, compared to the odds of the outcome occurring in the absence of that exposure.

Question 8

External validity of a study refers to

Answer 8

The degree to which the conclusions in a study would hold for other persons in other places and at other times. The external validity of a study refers to the generalisability of the research findings beyond the immediate study sample. It considers whether the results are applicable to other groups, settings, or time periods.

Question 9

Reliability

Answer 9

Reliability is the extent to which an experiment, test, or any measuring procedure yields the same result on repeated trials.

Question 10

Internal validity

Answer 10

Internal validity is concerned with how well an experiment is done, especially whether it avoids confounding (more than one possible independent variable [cause] acting at the same time). It’s about ensuring that any observed effects are due to manipulation of the independent variable and not some other factor.

Question 11

Validity

Answer 11

Validity in research refers to the extent to which a tool measures what it is intended to measure.

Question 12

the GRADE approach is used for what purpose

Answer 12

The GRADE approach (Grading of Recommendations Assessment, Development and Evaluation) is a system used for rating the quality or certainty of evidence in systematic reviews, health technology assessments (HTA), and clinical practice guidelines. It provides a transparent framework for developing and presenting summaries of evidence, primarily in the form of systematic reviews and guidelines. The GRADE approach reflects how confident we can be that the effect estimates are correct.

Question 13

Absolute risk reduction

Answer 13

The absolute risk reduction (ARR) is the calculated difference between the risk of an outcome in the exposed group and the unexposed group.

Absolute risk reduction = (Control event rate) - (Experimental event rate)

Question 14

Number Needed To Treat

Answer 14

Numbers needed to treat (NNT) is a measure that indicates how many patients would require an intervention to reduce the expected number of outcomes by one.

It is calculated by 1/(Absolute risk reduction) and is rounded to the next highest whole number.

Question 15

Drug development phases

Answer 15

Phase 1:
small studies (e.g. 100) on HEALTHY volunteers.
used to assess pharmacodynamics and pharmacokinetics.

Phase 2:
small studies (e.g. 100-300) on actual patients.
examines efficacy, adverse effects.

Phase 3:
Larger studies (e.g. 500-5,000 patients).
Examines efficacy, adverse effects
may compare drug with existing treatments.
Studies of special groups e.g. renal, elderly.

If drug shown to be safe and effective then drugs may be approved for marketing

Phase 4:
Post-marketing surveillance

Question 16

Wilcoxon signed-rank test

Answer 16

Compares two sets of observations on a single sample, e.g. a ‘before’ and ‘after’ test on the same population following an intervention.

Question 17

Recall bias

Answer 17

Difference in the accuracy of the recollections retrieved by study participants, possibly due to whether they have disorder or not. E.g. a patient with lung cancer may search their memories more thoroughly for a history of asbestos exposure than someone in the control group. A particular problem in case-control studies.

Question 18

Publication bias

Answer 18

Failure to publish results from valid studies, often as they showed a negative or uninteresting result. Important in meta-analyses where studies showing negative results may be excluded.

Question 19

Lead-time bias

Answer 19

Occurs when two tests for a disease are compared, the new test diagnoses the disease earlier but there is no effect on the outcome of the disease.

Question 20

Hawthorne effect

Answer 20

Describes a group changing it’s behaviour due to the knowledge that it is being studied

Question 21

research types would you regard most highly?

Answer 21

Meta-analysis is the systematic review of randomised controlled trials. This is the highest level of evidence.

Question 22

What is heterogeniety?

Answer 22

The heterogeneity is a measure of how different the studies included in the meta-analysis are. Ideally they should be as similar as possible and any difference in effect should be due to sampling error alone. However, in reality, studies done at different times in different places may be very different. Using Higgins I2, a value of 0% indicates statistical homogeneity. Significant statistical heterogeneity is often considered to be present if I2 is 50% or more.

Question 23

Types of Study?

Answer 23

Cohort studies are prospective and observational. The objective is to test aetiological hypotheses. With cohort studies groups of people are studied over a period of time with respect to a given disease. The comparison between groups is with respect to disease e.g. following groups of smokers and non-smokers in time to see whether one group is more prone to cancer. This is a good method for determining the incidence and natural history of a condition and avoids the ethical issues that plague randomised control trials. Cohort studies also allow the calculation of the effect of each variable on the probability of developing the outcome of interest (relative risk).

Case-control studies are retrospective and observational. Characteristics of subjects with a disease are compared with a selected group of control subjects without the disease. The comparison between groups is with respect to exposure, e.g. looking at groups of individuals with and without cancer to see if features in their past might explain a causal route for the cancer. Case-control studies can show associations but cannot establish causality.

In a crossover study each subject receives treatment and placebo in a random order. Each subject thus acts as his own control. Crossover studies are only suitable for evaluating palliative treatments of chronic stable conditions, e.g. assessing a new mucolytic therapy in cystic fibrosis patients and comparing this to current best treatment.

Cross-sectional studies describe characteristics of a population. Data is collected at one point in time and the relationships between characteristics are considered. An example would be to see how many of a particular group of people drink alcohol, and how many of that group have liver cirrhosis.

Systematic reviews and meta-analyses review published literature from previously well-conducted trials. The results are then pooled, and the data is re-analysed.

Question 24

Lead time bias

Answer 24

Lead time bias occurs when two tests for a disease are compared and although one of the tests diagnoses the disease earlier, this does not translate in to a survival benefit (i.e. it does not affect the outcome of the disease).

The screening test may appear to prolong survival but it actually only results in an earlier diagnosis. Morbidity (including patient anxiety and potential treatment side-effects) is increased as the patient must live with the knowledge of the disease for longer.