Definition of a few Terms in Educational Testing
By Inderbir Kaur Sandhu, Ph.D
Q:
What do the following Educational Testing Terms mean:
NCE
Critical Value
Expected Difference
Base Rate
Grade Equivalents
How are they determined?
How to interpret them?
Is Age Equivalents more reliable than Grade Equivalents?
Thank You!
A: I will try to explain the
terms above as simply as possible, but it can get quite
technical.
Normal curve equivalent (NCE) is a score based on the
percentile rank. It indicates the position a student falls
on a normal curve (a symmetrical curve/bell curve
representing the normal distribution). This enables us to
determine a student's rank compared to other students on the
same test. Because the percentile rank scale is not an
equalinterval scale (scale in which different scores
represent an ordering, for e.g., from highest to lowest, and
in which all scores along the scale represent the same
interval). This means the difference between any two scores
is not the same between any other two scores or The
difference between two adjacent scores has the same meaning
across the scale. This feature makes NCEs useful for
comparisons between different tests. In short, NCEs are
equalinterval scale conversions of percentile ranks. In
educational testing, students who progress in the grade
levels for example, will have a net gain in the NCE score
(which means they have made progress in comparison to the
general population) those while whose who show less progress
would indicate a net loss in their NCE ranks.
Critical Value is used in significance testing.
Significance determines if an observed value of a statistic
differs enough from a hypothesized value (null hypothesis)
of a parameter to draw the inference that the hypothesized
value of the parameter is not the true value. It is the
value that a test statistic must exceed in order for the
null hypothesis to be rejected.
Expected Difference is any difference based on the
average (mean) that is expected from two groups. For
example, on an achievement test score, the results indicate
that there was no expected difference in mean achievement
test scores between group 1 (say, a group that was
treated/taught with special learning methods) and for
another group that was not treated. The difference is
usually based on a significance level of 0.05.
Base Rate is the proportion of students in the
population under study who exhibit characteristics being
measured by the test. For example in an ability test, the
level of criterion performance necessary for someone to be
considered successful is determined. Hence, the proportion
of all testtakers who would be considered successful is
called the base rate.
Grade Equivalent scores determine performance in
terms of theoretical level of education. It shows a child's
actual performance on a test that is the number answered
correctly (raw score) can be converted to a Grade Equivalent
score. The Grade Equivalent score expresses the grade level
of students who on average get that raw score. So, for
example, if a 3rd grade child who is tested achieves a raw
score of 10 points, and children near the end of 1st grade
(say, at the 9th month) on average earn a raw score of 10
points, the 3rd grade child will be assigned a Grade
Equivalent score of 19. “Grade Equivalent scores are based
on the assumption that it is helpful to define progress in
terms of the gradelevel at which an average student attains
a given level of knowledge or skill.” (www.ets.org/letstalk,
a very interesting read for parents on testing). Grade
Equivalent scores are typically only used in primary and
secondary education.
Age Equivalent scores shows the typical age of the
norm group that obtained a similar score. Similar to the
Grade Equivalent Scores, Age Equivalent Scores allows for
comparison of the child’s scores with those of others who
were tested on the same test. Age Equivalent Scores have the
same limitations as Grade Equivalent Scores. The reliability
both age and grade equivalent scores is limited by the
relationship between the equivalents and the raw scores on
which they are based. An age or grade equivalent is simply
the median raw score for a particular age or grade level.
For example, a test that measures vocabulary generally
occurs more during elementary years. Therefore, the raw
scores increase at a greater rate with younger examinees
than with older examinees. Therefore, a similar change in
raw scores of younger examinees and of older examinees will
be represented quite differently in age equivalent scores.
This causes the reliability for ageequivalent scores much
poorer for advanced testtakers.
Hopefully the above is helpful.
