Reliability & validity are terms that refer to the consistency and accuracy of a quantitative measurement questionnaire, technical device, ruler, or any other measuring device.  It means that the outcome measure can be trusted and is relatively error free.

• Reliability – This means that the instrument measures CONSISTENTLY
• Validity – This means that the instrument measures ACCURATELY. In other words it measures what it is supposed to measure and not something else.

For example: If your bathroom scale measures weight, then it is a valid measure of weight (e.g. it doesn’t measure BP or stress). You might say it had high validity. If your bathroom scale measures your weight as the same thing when you step on and off of it several times then it is measuring weight reliably  or consistently; and you might say it has high reliability.