Much more typical is the measurement where
the XO score differences also reflect (Selltiz et al.,
1976, p. 164-8):
1. True differences in other relatively stable characteristics
which affect the score, e.g., a person's
willingness to express his or her true feelings.
2. Differences due to transient personal factors, e.g.,
a person's mood, state of fatigue.
3. Differences due to situational factors, e.g., whether
the interview is conducted in the home or at a central
facility.
4. Differences due to variationsi n administratione, .g.,
interviewers who probe differently.
5. Differences due to sampling of items, e.g., the
specific items used on the questionnaire;i f the items
or the wording of those items were changed, the
XO scores would also change.
6. Differences due to lack of clarity of measuring
instruments, e.g., vague or ambiguous questions
which are interpreted differently by those responding.
7. Differences due to mechanical factors, e.g., a check
mark in the wrong box or a response which is coded
incorrectly.
Not all of these factors will be present in every
measurement, nor are they limited to information
collected by questionnaire in personal or telephone
interviews. They arise also in studies in which self-administered
questionnaires or observational techniques
are used. Although the impact of each factor on the
XO score varies with the approach, their impact is
predictable. They distort the observed scores away
from the true scores. Functionally, the relationship
can be expressed as:
XO= XT + X + XR
where:
Xs = systematic sources of error such as stable characteristics
of the object which affect its score,
and
XR = random sources of error such as transient personal
factors which affect the object's score.
A measure is valid when the differences in observed
scores reflect true differences on the characteristic
one is attempting to measure and nothing else, that
is, XO = XT. A measure is reliable to the extent that
independent but comparable measures of the same
trait or construct of a given object agree. Reliability
depends on how much of the variation in scores is
attributable to random or chance errors. If a measure
is perfectly reliable, XR = 0. Note that if a measure
is valid, it is reliable, but that the converse is not
necessarily true because the observed score when
XR = 0 could still equal XT + X,. Thus it is often
said that reliability is a necessary but not a sufficient
condition for validity. Reliability only provides negative
evidence of the validity of the measure. However,
the ease with which it can be computed helps explain
Much more typical is the measurement wherethe XO score differences also reflect (Selltiz et al.,1976, p. 164-8):1. True differences in other relatively stable characteristicswhich affect the score, e.g., a person'swillingness to express his or her true feelings.2. Differences due to transient personal factors, e.g.,a person's mood, state of fatigue.3. Differences due to situational factors, e.g., whetherthe interview is conducted in the home or at a centralfacility.4. Differences due to variationsi n administratione, .g.,interviewers who probe differently.5. Differences due to sampling of items, e.g., thespecific items used on the questionnaire;i f the itemsor the wording of those items were changed, theXO scores would also change.6. Differences due to lack of clarity of measuringinstruments, e.g., vague or ambiguous questionswhich are interpreted differently by those responding.7. Differences due to mechanical factors, e.g., a checkmark in the wrong box or a response which is codedincorrectly.Not all of these factors will be present in everymeasurement, nor are they limited to informationcollected by questionnaire in personal or telephoneinterviews. They arise also in studies in which self-administeredquestionnaires or observational techniquesare used. Although the impact of each factor on theXO score varies with the approach, their impact ispredictable. They distort the observed scores awayfrom the true scores. Functionally, the relationshipcan be expressed as:XO= XT + X + XRwhere:Xs = systematic sources of error such as stable characteristicsof the object which affect its score,andXR = random sources of error such as transient personalfactors which affect the object's score.A measure is valid when the differences in observedscores reflect true differences on the characteristicone is attempting to measure and nothing else, thatis, XO = XT. A measure is reliable to the extent thatindependent but comparable measures of the sametrait or construct of a given object agree. Reliabilitydepends on how much of the variation in scores isattributable to random or chance errors. If a measureis perfectly reliable, XR = 0. Note that if a measureis valid, it is reliable, but that the converse is notnecessarily true because the observed score whenXR = 0 could still equal XT + X,. Thus it is oftensaid that reliability is a necessary but not a sufficientcondition for validity. Reliability only provides negativeevidence of the validity of the measure. However,the ease with which it can be computed helps explain
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