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Validity and Reliability


We typically use a statistical software program SPSS (Statistical Package for ... about reliability and validity refer to instrument development. ... Testing ... – PowerPoint PPT presentation

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Title: Validity and Reliability

Validity and Reliability
Instrument Concerns
  • Validity Does the instrument measure what it is
    supposed to measure?
  • Reliability Does the instrument consistently
    yield the same results?

  • There are three major categories of validity.
  • Content (also called Face Validity)
  • Criterion Related
  • Concurrent
  • Predictive
  • Construct

Content Validity
  • Researchers are most concerned with Content or
    Face Validity
  • Does the instrument really measure what it is
    supposed to measure.
  • This is similar to does a test in a class really
    cover the content that has been taught.
  • Content validity is determined by having a panel
    of experts in the field examine the instrument
    and compare it to the research objectives.

Concurrent Validity
  • Does the instrument yield similar results to
    other recognized instruments or tests?
  • Example If I developed an instrument to identify
    quality FFA chapters and most of the chapters I
    identified were recognized as 3 star FFA chapters
    by the National FFA organization, this means my
    instrument has concurrent validity.

Predictive Validity
  • Does the instrument predict how well subjects
    will perform at a later data?
  • Example I developed an instrument that I
    believed would identify freshman in high school
    who would go to college. Four years later if the
    students I identified did go to college, then my
    instrument has predictive validity.

Construct Validity
  • Does the instrument really measure some
    abstract or mental concept such as
    creativeness, common sense, loyalty, or honesty.
  • This is very difficult to establish, thus we
    seldom are concerned with construct validity in
    educational research.

  • An instrument first must be valid, then it must
    be reliable.
  • It must measure accurately and consistently each
    time it is used.
  • If I had a bathroom scale and stepped on it
    three times in a row, and got drastically
    different weights each timethen it wouldnot be
    reliable. Some survey instruments that are not
    well designed behave in the same manner.

Determining Instrument Reliability
  • Test-Retest administer the instrument, then
    administer it to the same group later, correlate
    the two scores (I really dont like this
    technique. I think it is impractical and too time

Determining Instrument Reliability
  • Equivalent Forms have two forms of the
    instrument, administer form A to a group then
    administer form B later, correlate the two scores
    (I really dont like this technique. I think it
    is impractical and too time consuming)

Determining Instrument Reliability
  • Split-halves divide the instrument in half and
    calculate a correlation between the halves. There
    should be a high correlation. This determines
    internal consistency which is generally
    regarded as being synonymous with reliability.

Determining Instrument Reliability
  • Statistical calculations There are several
    statistical procedures for determining internal
  • Cronbachs Alpha
  • Kuder-Richardson 20 or 21
  • Reliability coefficients (scores) range from 0-1.
    The higher the score, the more reliable the
    instrument. Aim for .70 or higher.

Note Technically there is a difference between
reliability and internal consistency but for
all practical purposes they are interchangeable
  • We typically use a statistical software program
    SPSS (Statistical Package for the Social
    Sciences) to determine internal consistency of an
    instrument. NCSU students can download this for
    free from the university software site.

Increasing Instrument Reliability
  • In addition to doing a good job in designing an
    instrument, two factors impact on the reliability
    of an instrument
  • Number of questions the more questions the more
    reliable (but more questions can reduce response
  • Number of people completing the instrument the
    more who take it, the more reliable it will be

  • Which two images depict reliability?
  • Which image shows both reliability and validity?
  • Which image shows neither reliability or validity?

Internal Validity
  • The previous slides about reliability and
    validity refer to instrument development. There
    is another type of validity that researchers are
    concerned with and that involves experimental
    and quasi-experimental research. This other
    validity is known as Internal Validity

What is Internal Validity?
  • If we are sure the treatment made a difference in
    an experiment, then we believe the study in
    internally valid.
  • In other words the independent variable made the
    difference and not some other variables.

Common threats to internal validity
  • History
  • Maturation
  • Testing
  • Instrumentation
  • Statistical Regression
  • Experimental Mortality
  • Selection

  • the specific events which occur between the
    first and second measurement.
  • External events might influence the outcome of
    the study, not the treatment.
  • Assume I developed a teaching unit to make people
    more accepting of other cultures and started the
    use of this unit on Sept. 1, 2001 and ended the
    unit on Sept. 30, 2001.
  • To my great surprise, my students were less
    accepting of other cultures after completing the
    unit. Why? Is my teaching unit no good?
  • The 9/11 terrorist attack occurred during this
    time period and probably had a major impact on
    the attitudes of my students regarding other

  • the processes within subjects which act as a
    function of the passage of time. i.e. if the
    project lasts a few years, most participants may
    improve their performance regardless of
  • I develop a liquid that children can drink once a
  • This liquid is designed to help potty train
  • If they start drinking it at the age of 1, 98 of
    the children are potty trained by the age of 3.
  • Did my treatment make the difference or do
    children naturally become potty trained during
    this time period?

  • When you have a pretest and a posttest, students
    may remember some of the questions from the
    pretest and do better on the posttest because of
  • If the time between the pre and posttest is
    short, this could be a problem.

  • To prevent the testing threat to the internal
    validity of research, some researchers will
    develop another version of the test.
  • Version A will be used as a pretest.
  • Version B will be the posttest.
  • It is possible the two versions of the test are
    not of the same difficulty and may have other
  • If we use interviewers or observers to collect
    data, they may become tired after a while and
    report things differently than they did when they
    were fresh.

Statistical Regression
  • Also known as regression to the mean.
  • This threat is caused by the selection of
    subjects on the basis of extreme scores or
  • People who score unusually high or low on a
    measure typically have more normal scores on
    subsequent tests.
  • Give me the 20 students with the lowest reading
    scores and I guarantee that they will show
    immediate improvement right after my reading

Experimental Mortality
  • The loss of subjects during the study
  • Those who stay in a study all the way to the end
    may be more motivated to learn and thus achieved
    higher performance.

  • If you do not randomly select subjects for the
    experimental group and the treatment group, you
    may be introducing a bias.
  • There are two groups of senior ag students. One
    meets 1st period and one meets 4th period.
  • One group gets the treatment, the other group is
    the control group.
  • Advanced math is taught in this school during 1st
    period, thus all the smarter students, including
    the Ag seniors, are in this class.
  • So by comparing the scores of the 1st and 4th
    period students you are really comparing smarter
    students with average students.

John Henry or Hawthorne Effect
  • People know they are in a study, so they try
    extra hard to do good.

Controlling these Threats
  • Typically these threats to the internal validity
    of research can be controlled by
  • Having a control group
  • Randomly assigning people to groups and randomly
    assigning treatments to groups.

Background Information
  • Campbell and Stanley are the researchers who
    identified and described the threats to the
    internal validity of research.