1. Precision, Accuracy, And Validity
Precision:- a very precise measurement is a one that has nearly the same value each time it is measured. A scale measurement is more precise than a qualitative one.
The more precise a measurement the greater the statistical power.
Assessing precision: usually by S.D.of repeated measurements, also the Co.V. or correlation coefficient.

2. Precision is usually affected by random errors, there are 2 main sources of errors:
Subjective:- subject variability due to intrinsic biological variability ex.: fluctuation in mood, B.Pr., heart rate…
Objective :
1- Observer: hand-eye coordination
2- Instruments: due to fluctuating environmental factors such as temperature, background noise,…

3. Strategies For enhancing precision:
Through decreasing random errors, and this is done by:
1- Standardizing the measurement method
2- Training & certifying the observer
3- Refining the instruments
4- Automating the instruments
5- Repetition: repeat the measurement & use the mean

4. Accuracy
The accuracy of a variable is the degree to which it actually represents what it is intended to represent.
Precision
Accuracy
Definition
Best way to use
threatened by
The degree to which a variable has nearly the same value when measured several times.
Comparison among repeated measures
Random error( chance) subjective and objective
The degree to which a variable actually represents what it is supposed to
comparison with a reference standard
systematic error (bias)
subjective & objective

5. Accuracy is affected by systematic errors (bias) of two types:
1- Subjective: Co-operation, re-call, illiteracy…
2- Objective :
-observer: conscious or un-conscious
-Instruments: which are not calibrated recently. .

6. Strategies for enhancing accuracy:
The same 1-4 strategies of precision and :
5- Making unobtrusive measures: design measurements that the observer is unaware of.
6- Blinding: Single, double, triple (the researcher or the manager who is reading the outcome)
7- Calibrating the instruments
The discussion on how vigorously to peruse each of these strategies depends on feasibility and cost of the strategy

7. Validity
Predictive Validity: the degree to which the measurement successfully predicts an outcome
Criterion related Validity: the degree to which the measurement agrees with other approaches for measuring the same characteristics (ex. Question about stress)
Face Validity: a subjective judgment whether a measurement makes sense intuitively, whether it seems to be a reasonable approach.