and Fishbein (2005)
discussed factors that may affect the BI / behavior relation
and, therefore, should be taken into account when constructing
BI measures. First, is aggregation. As with
most constructs, indices of BI are most reliable and have
the highest predictive validity when they include multiple
items. Thus, if the relevant behavior is diet, the criterion
and the BI measures should both include different variations
of the focal construct (e.g., eat fruits and vegetables, monitor
fat intake, avoid sweets). Second is the principle of compatibility,
which states that the BI and behavioral measures should "involve
exactly the same action, target, context, and time." (p.
26). Thus, a more global or abstract intention—"I
intend to drive safely" may not accurately predict a specific
behavior, such as wearing seat belts. Another factor is commitment.
If the behavior (goal) is important to the individual, his/her
expressed intention to do it should relate more strongly to
its performance. Commitment and strength of intention are
likely to be correlated, however, (Rhodes
& Matheson, 2005);
so measuring commitment may be redundant with assessment of
Although BI does have very good predictive validity, it is
still the case that it doesn't explain 70% to 80% of the variance
in health behavior, which raises the methodological question
(with theoretical implications) of why? One factor has to
do with stability. Conner
, Sheeran, Norman, and Armitage (2000) reported that
health screening (Study 1) and maintaining a low fat diet
(Study 2) were better predicted by intentions when those intentions
were relatively stable across a one-year period of time (cf.
& Sheeran, 2004).
Related to this issue, another obvious factor is the time
lag between measurement of BI and behavior. Although
it varies by behavior (and age of the respondent), generally
speaking, the BI - behavior relation tends to diminish when
the measurement gap between the two exceeds a few months (Sheeran
& Orbell, 1998). Another complicating factor is emotion.
When asked to report intention to engage in a particular behavior-get
a mammogram, for example, or a colonoscopy-one might not consider
(or fully appreciate) the level of anxiety over getting tested
that, in classic approach-avoidance terms, might inhibit behavior
at the time of execution. The same applies to interference
that may come from the ingestion of substances at the time
of performance (Ajzen
& Fishbein, 2005).