1. Reliability and Validity

2. Measurement Error
Whatever measurement we might make with regard to some psychological construct, we do so with some amount of error
Any observed score for an individual is their true score with error added in
There are different types of “error”, but here we are concerned with a measure’s inability to capture the true response for an individual
Observed Score = True score + Error of measurement

3. Reliability
Reliability refers to a measure’s ability to capture an individual’s true score, i.e. to distinguish accurately one person from another
While a reliable measure will be consistent, consistency can actually be seen as a by-product of reliability, and in a case where we had perfect consistency (everyone scores the same and gets the same score repeatedly), reliability coefficients could not be calculated
No variance/covariance to give a correlation
The error in our analyses is due to individual differences but also the lack of the measure being perfectly reliable

4. Reliability Criteria of reliability Test-retest reliability Issues
Test components (internal consistency)
Test-retest reliability
Consistency of measurement for individuals over time
The score similarly e.g. today and 6 months from now
Issues
Memory
If too close in time the correlation between scores is due to memory of item responses rather than true score captured
Chance covariation
Any two variables will always have a non-zero correlation
Reliability is not constant across subsets of a population
General IQ scores good reliability
IQ scores for college students, less reliable
Restriction of range, fewer individual differences

5. Internal Consistency
We can get a sort of average correlation among items to assess the reliability of some measure1
As one would most likely intuitively assume, having more measures of something is better than few
It is the case that having more items which correlate with one another will increase the test’s reliability
1. Cronbach’s alpha is a function of the number of items and average correlation among them

6. What’s good reliability?
While we have conventions, it really kind of depends
As mentioned reliability of a measure may be different for different groups of people
What we may need to do is compare reliability to those measures which are in place and deemed ‘good’ as well as get interval estimates to provide an assessment of the uncertainty in our reliability estimate
Note also that reliability estimates are biased upwardly and so are a bit optimistic
Also, many of our techniques do not take into account the reliability of our measures, and poor reliability can result in lower statistical power i.e. an increase in type II error
Though technically increasing reliability can potentially also lower power1
1. We don’t really want to go there, but you can see the related notes on the 5710 page. It’s at the end of the ‘Experimental design’ notes.

7. Replication and Reliability
While reliability implies replicability, assessing reliability does not provide a probability of replication
Note also that statistical significance is not a measure of reliability or replicability1
Replication is not perhaps conducted as much as should be in psychology for a number of reasons
Practical concerns, lack of publishing outlets etc.
Furthermore, knowing our estimates are biased and variable themselves, we might even think that in many cases we would not expect consistent research findings
In psychology, many people spend a lot of time debating back and forth about the merits of some theory, citing cases where it did or did not replicate
However the lack of replication could be due to low power, low reliability, problem data, incorrectly carrying out the experiment etc.
In other words, we didn’t repeat because of methodology, not because the theory was wrong
1. Often you may see people use the term ‘statistically reliable’ meaning statistically significant. Please don’t. It is grossly misleading terminology and, taken literally, usually wrong.

8. Factors affecting the utility of replications
You can’t step in the same river twice!
Heraclitus1
When
Later replications are not providing as much information, however they can contribute greatly to the overall assessment of an effect
Meta-analysis
How
There is no perfect replication (different people involved, time it takes to conduct etc.)
Doing ‘exact’ replication gives us more confidence in the original finding (should it hold), but may not offer much in the way of generalization
Example: doing a gender difference study at UNT over and over. Does it work for non-college folk? People outside of Texas?
1. His student Cratylus said that you couldn’t step in the river even once.

9. Factors affecting the utility of replications
By whom
It is well known that those with a vested interest in some idea tend to find confirming evidence more than those that don’t
Replications by others are still being done by those with an interest in that research topic and so may have a ‘precorrelation’ inherent in their attempt
Direct: correlation of attributes of persons involved
Indirect: correlation of data to be obtained
Gist, we can’t have truly independent replication attempts, but must strive to minimize bias
The more independent replication attempts are, the more informative they will be

10. Validity
Validity refers to the question of whether our measurements are actually hitting on the construct we think they are
While we can obtain specific statistics for reliability (even different types), validity is more of a global assessment based on the evidence available
We can have reliable measurements that are invalid
Classic example: The scale which is consistent and able to distinguish from one person to the next but actually off by 5 pounds

11. Validity Criteria in Psychological Testing
Content validity
Criterion validity
Concurrent
Predictive
Construct-related validity
Convergent
Discriminant
Items represent the kinds of material (or content areas) they are supposed to represent
Are the questions worth a flip in the sense they cover all domains of a given construct?
E.g. job satisfaction = salary, relationship w/ boss, relationship w/ coworkers etc.

12. Validity Criteria in Psychological Testing
Criterion validity
the degree to which the measure correlates with various outcomes
Does some new personality measure correlate with the Big 5
Concurrent
Criterion is in the present
Measure of ADHD and current scholastic behavioral problems
Predictive
Criterion in the future
SAT and college gpa

13. Validity Criteria in Psychological Testing
Construct-related validity
How much is it an actual measure of the construct of interest
Convergent
Correlates well with other measures of the construct
Depression scale correlates well with other dep scales
Discriminant
Is distinguished from related but distinct constructs
Dep scale != Stress scale

14. Validity Criteria in Experimentation
Statistical conclusion validity
Is there a causal relationship between X and Y?
Correlation is our starting point (i.e. correlation isn’t causation, but does lead to it)
Related to this is the question of whether the study was sufficiently sensitive to pick up on the correlation
Internal validity
Has the study been conducted so as to rule out other effects which were controllable?
Poor instruments, experimenter bias
External validity
Will the relationship be seen in other settings?
Construct validity
Same concerns as before
Ex. Is reaction time an appropriate measure of learning?

15. Summary
Reliability and Validity are key concerns in psychological research
Part of the problem in psychology is the lack of reliable measures of the things we are interested in1
Assuming that they are valid to begin with, we must always press for more reliable measures if we are to progress scientifically
This means letting go of supposed ‘standards’ when they are no longer as useful and look for ways to improve current ones
1. Always, always, always find the reliability estimates for the measure you are about to use. The information is readily available in original and related articles (always go to the original though), Mental Measurement Yearbook etc. At least give yourself a chance to do a decent study by using reliable measures.