In everyday life we usually try to get the facts before we make a decision.  The greater the amount of accurate information we can gather, the more confident we can be that we are making the right decision.  The same thing holds true in the classroom when we assess students to get information that's used to decide what they know and can do, to diagnose their needs, to decide if we need to teach something in a different way, to plan instruction, to assign grades, to communicate with parents, and to guide us in the development of School Improvement plans.  The consequences of making a poor decision can be very serious, so it's important to make sure that we're basing our decisions on an adequate amount of sound information.  Or to put it another way we need assessment results that are both reliable and valid for our particular purpose.

An assessment is reliable if it yields results that are accurate and stable.  In order for a Performance Assessment to be reliable, it should be administered and scored in a consistent way for all the students who take the assessment.

An assessment is valid for a particular purpose if it in fact measures what it was intended to measure.  An assessment of a learning outcome is valid to the extent that scores truly measure that outcome and are not affected by anything irrelevant to the outcome.

Some important aspects of Validity are content coverage, generalizability and fairness.  The assessments for a given outcome should be aligned with the both the outcome and instruction and, when taken together, should cover all important aspects of the outcome.  The assessments should address the higher-order thinking skills specified in the outcome.  The tasks used should have answers or solutions that can't be memorized, but which, instead, call on the student to apply knowledge and skills to a new situation. 

Assessment results are generalizable [to the extent that] if there is evidence that scores on one assessment can predict how well students perform on another assessment of the same outcome. (Chicago Public Schools Instructional Intranet: http://intranet.cps.k12.il.us/Assessments/Ideas_and_Rubrics/Intro_
Scoring/Reliable_Assessments/reliable_assessments.html