Trial size design and study population

A clinical trial must obviously have a control group, against which the test (intervention) group can be compared. The control group may receive: (a) no intervention at all; (b) a placebo (i.e. a substance such as saline, which will have no pharmacological or other effect); (c) the therapy most commonly used at that time to combat the target disease/condition. The size of the trial will be limited by a number of factors, including:

• economic considerations (level of supporting financial resources);

• size of population with target condition;

• size of population with target condition after additional trial criteria have been imposed (e.g. specific age bracket, lack of complicating medical conditions, etc.);

• size of eligible population willing to participate in the trial.

Whereas a comprehensive phase III trial would normally require at least several hundred patients, smaller trials would suffice if, for example:

• the target disease is very serious/fatal;

• there are no existing acceptable alternative treatments;

• the target disease population is quite small;

• the new drug is clearly effective and exhibits little toxicity.

Choosing the study population is obviously critical to adequate trial design. The specific criteria of patient eligibility should be clearly predefined as part of the primary question the trial strives to answer.

A number of trial design types may be used (Table 4.5), each having its own unique advantages and disadvantages. The most scientifically pure is a randomized, double-blind trial (Box 4.3). However, in many instances, alternative trial designs are chosen based on ethical or other grounds. In most cases, two groups are considered: control and test. However, these designs can be adapted to facilitate more complex subgrouping. Clinical trial design is a subject whose scope is too broad to be undertaken in this text. The interested reader is referred to the 'Further reading' section at the end of this chapter.

Table 4.5 Some clinical trial design types. Additional information may be sourced from appropriate references provided in the 'Further reading' section

Randomized control studies (blinded or unblinded)

Historical control studies (unblinded)

Non-randomized concurrent studies (unblinded)

Cross-over trial design

Factorial design

Hybrid design

Large simple clinical trials

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