Using Professional Judgment to Develop a Sampling Design
Using professional judgment does not mean you must take judgmental samples. Implementation of a judgmental sampling design should not be confused with the application of professional judgment (or the use of professional knowledge of the study site or process). Professional judgment should always be used to develop an efficient sampling design, whether that design is judgmental or probability-based. For more information on when to use judgmental sampling or a probabilistic sampling design, see Selecting a Sampling Design.
Merits of Judgmental vs. Probabilistic Sampling
| Judgmental Sampling | Probabilistic Sampling |
|---|---|
| Selection of samples based on professional judgment and prior information |
Selection of samples has a random component and may incorporate professional judgment and prior information |
| Should only be used to take a few samples (making it less expensive) | Needs an adequate number of samples to support performance objectives (making it more expensive) |
| Can be cost-effective with expert knowledge | Can be cost-effective with expert knowledge |
| Cannot reliably evaluate precision of estimates – does not allow the level of confidence in the results to be accurately quantified | Can evaluate precision of estimates – allows the level of confidence of the investigation to be accurately quantified |
| Personal judgment is used to interpret data relative to study objectives | Statistical inference is used to draw inferences from the data to the entire area/process. |
| Results are biased | Protects against bias |
| Easy to implement | Can be complicated to implement both in selecting samples and performing computations |
| Useful for emergency situations that need immediate sampling | Useful for situations where the level of confidence of the final result should be accurately quantified. |