Reliability and Validity

                     

 

Testing Reliability of a Scale à consistency à obtain same results on repeated occasions
Cronbach's Alpha	Measures components of scale Eg: the 6 questions that are added together to make a "Environmental Footprint" scale   Cronbach's Alpha based on Standardised Items   Mean  - range of means across the whole range of the scale Std Dev – higher variance – ie about std dev of one Item –Total Statistics ·         Scale mean if item deleted ·         Scale variance if item deleted ·         Corrected Item-Total Correlation o   higher à good o   lower  à bad à delete item ·         Cronbach's Alpha if Item Deleted o   look where alpha is higher than overall alpha
Intra Class Correlation Coefficient	Measure of Consistency Pearson's R cannot detect systematic error in scores (eg, practice effects ) Used in ·         test / retest ·         alternative forms ·         split half
Testing Validity of Scale  à How well scale measures what it is intended to measure
Content Validity	qualitative assessment
face validity
Criterion Validity - correlation	·         Concurrent validity à compare new test with existing test  : old & new score ·         Predictive validity à scores and secondary outcome
Construct validity - correlation	·         Convergent validity ·         discriminant (unrelated) validity
Incremental Valdity