Date: Thu, 6 Feb 1997 19:05:10 -0500
Reply-To: James Karr <jkarr@INTERLOG.COM>
Sender: "SPSSX(r) Discussion" <SPSSX-L@UGA.CC.UGA.EDU>
From: James Karr <jkarr@INTERLOG.COM>
Organization: InterLog Internet Services
Subject: Orthogonal designs & duplicate cards
I9ve got a question regarding the generation of orthogonal designs for
conjoint/trade-off research, and the issue of duplicate cards.
SPSS9s conjoint module can generate 3decks2 with the minimum number of
cards required to estimate the utility of each factor. It may also, from
time to time, produce decks with one or more duplicates (i.e., two cards
will have the identical combination of levels for all factors) as part of
the orthogonal design. When this happens, it alerts you to the presence
of such cards, and allows you to remove them or to generate a new deck.
Using an example, now let me ask: If 3 duplicate cards appear as part of a
16-card deck, should they be...
(a) deleted, because they contain a redundant combination of levels that
do not aid in estimating utilities;
(b) retained, because to delete them would remove almost 20% of the deck,
compromising its orthogonality;
or should the researcher (c) simply generate another 16-card deck, and
continue to do so until producing a deck with 16 unique cards?
To put the question another way: If a 16-card deck has 3 duplicates, does
that mean the orthogonal design *really* only needs 13 cards to estimate
the utilities for each factor?
Any comments would be appreciated.