scale scores at Time 2 can be predicted by Time 1 scores. Some critics of
the stability position have asked about the remaining 50%: Isn’t it fair tosay that there are equal amounts of stability and change?
This argument would be sound if personality scales were perfect in -
dicators of personality . But even the best instruments are fallible. When
a subject encounters the NEO-PI item, “I really like most people Imeet,” he or she must choose among five options, from “strongly dis -
agree” to “strongly agree.” If the subject happens to recall meeting sev -
eral obnoxious people lately , he or she may respond “disagree.” On a
second administration, the subject may place more weight on his or hergenerally friendly response to people and “agree” with the item. Thesubject’s personality has remained the same, but his or her reading ofthe item has changed. It is for this reason that scales combining manyitems are generally more accurate than those that include only a few:Random errors tend to cancel out when many items are used. (An illus-tration of this principle is seen in Table 13 [in Chapter 6] and Table 14,where the longer domain scales show higher stability coefficients thantheir component facet scales.)
There are also other sources of error in filling out questionnaires.
Respondents may be living through a difficult period, perhaps lookingfor work or adjusting to a divorce (Costa, Herbst, et al., 2000); theirtemporary mood may color their perception of themselves. Problemswith physical health may also affect responses. Whatever the causes,test scores tend to fluctuate from day to day for reasons that are unre-lated to personality change. Traditionally , scales are evaluated in part bytheir ability to give relatively constant or reliable scores despite these in -
fluences. This property is assessed by repeating the measure a few days
or weeks later for a sample of people and correlating the first and sec -
ond set of scores. Since it can be assumed that no true personality
change has occurred in the interval, retest correlations less than 1.0 canbe interpreted as evidence of unreliability . Personality scales normallyhave retest reliabilities in the range of .70 to .90.
In longitudinal studies, the correlations between the initial and
later administrations of the test are less than 1.0, lowered both by theday-to-day fluctuations of questionnaire responses and by whatevertrue change has occurred. Retest reliability puts an upper limit on sta -
bility: We cannot expect to find high correlations over a period of years
if we do not observe them over a period of days. It is therefore reason -
able to ask how close to this upper limit stability coefficients reach andQualifications and Rival Hypotheses 123