192 PSYCHOMETR1KA
formulate different PDFs for the latent responses
X i
conditional on Z. For example, this PDF
may involve statistical dependencies between the latent responses, reflecting the hypothesis that
the application of one mental operation influences the application of others. Second, one can
assume the latent responses to be polytomous, or even continuous, instead of dichotomous. For
example, continuous latent responses may be an appropriate choice for a detection or identifia-
bility task in which stimuli are presented that vary on a number of continuous dimensions. And
third, one can formulate other condensation rules besides the conjunctive one.
Another useful condensation rule is the
disjunctive
one. It is defined as follows:
K
C(Xi) = 1 - I~(1 -
Xik).
k=l
This function has the value 1 if and only if there is at least
one
Xik
that has the value 1. A
useful interpretation of this condensation rule is in terms of mental operations or
strategies
whose
successful application is
sufficient
for giving a correct response. Together with the assumption of
LSI of the
Xi~'s,
this condensation rule leads to the following form for P (Yi t Z; ~/i):
P(YilZ;
T/i) -~
1 - 17
P(Xik =
0lZk)
P(Xik
~- 01Zk)
k=l k=l
(7)
Still other condensation rules may have more than two different function values. For ex-
ample, one can formulate MCLCMs for multiple choice items by chosing the condensation rule
such that every pattern of latent responses is mapped into a particular response alternative accord-
ing to some hypothesis about the response process. Such an hypothesis should not only specify
how the correct response comes about, but also the different
incorrect
responses.
Estimation (section 4) will be considered only for the conjunctive and the disjunctive model
with independent latent responses. The extension to other condensation rules and other models
for the latent responses is straightforward, however.
Restrictions on the item parameters.
Besides extending the LRM-framework by formu-
lating other PDFs for the latent responses and using other condensation rules, the usefulness
of this class of models is also enhanced by introducing restrictions on the item parameters. In
particular, interesting special cases appear if
rlikO
and/or
l"]ikl are
fixed at 0 or 1. Under the con-
junctive condensation rule, fixing rlik0 at 0, the restriction is imposed that this item absolutely
requires mastery of this mental operation. This type of restrictions is very well suited for testing
hypotheses about the response process. For example, one can fix r/il0 at 0 for all fraction items
that require splitting, and fix r/i20 at 0 for all fraction items that require identifying. The introduc-
tory example was implicitely based on this kind of deterministic response model (see Figure 1).
Also under the conjunctive condensation rule, by fixing
both ~]ikO
and
17ikl
at
l,
the restriction
is imposed that the corresponding mental operation is simply not involved in the solution of this
item. For example, the fraction 2/6 .... does not involve splitting. So, one can fix rlil0 and T/ill
at 1 for this item. Under the disjunctive condensation rule, similar restrictions can be imposed.
Latent response and latent variable models.
At this point, we should point out the dif-
ference between LRMs and latent variable models in general. In a broad sense, LRMs
are
latent
variable models because the model for the observed data (the
Yi's)
is obtained by integrating
(summing) out a set of unobserved random variables (the Z's and the Xi's). In a narrow sense,
latent variable models (the factor analysis model, the latent class model) involve (a) a draw from
the PDF of the latent variables, and (b) a draw from the conditional PDF of the observed variables
given the latent variables. This does not hold for LRMs, because there is no conditional PDF of
observed variables given latent variables; latent variables are mapped into observed variables
by means of a
function.
This mapping of latent into observed random variables is the essential
new feature of LRMs, distinguishing it from classical latent variable models, and creating the