orthoganal + blocking => strange and deceiving design

[stratec]

Hello,

I’m new user of NGene. To simplify my task I started with an orthogonal design even if I understand that the NGene’s designers prefer efficient design for a lot of good reasons.

My intention was also to use the blocking procedure.

So, my code was the following:

Design
? Question TRAIN SP1b choix modal 3 alternatives pas de retard
;alts = Voiture, Train, TGV
;rows = 144
;orth = sim
;block = 12
;model :
U(Voiture) = b1 + b2 * TVP[1,2,3] + b3 * CVP[1,2,3] /
U(Train) = b20 + b4 * TTR[1,2,3] + b3 * PTR[1,2,3] + b6 * FRTR[1,2,3] /
U(TGV) =b10 * TTGV[1,2,3] + b3 * DELTA[1,2,3] + b12 * FRTGV[1,2,3]
$

I get a very surprising design with many repetitions of the same choice situation. Each block which should be constituted by 12 choice situations are actually constituted by 3 different choice situations which are, each, repeated 4 times.
I have some difficulties to figure out why I get this so strange (and deceiving) design.

Any help would be more than welcome

Tanguy ISAAC
Stratec SA

[Michiel Bliemer]

Orthogonal designs are still widely applied and that is why Ngene supports this. For pilot studies in which the analyst does not have any idea yet about priors, it could be a useful design to start with. But one always has to be careful in generating one and using one.

The same holds here. There exists an orthogonal design for estimating this model with 12 rows. Instead, you are asking for 144 rows. Blocking is typically used to decrease the number of choice tasks per respondent if the orthogonal design is too large. In this case, the orthogonal design is small and by inflating it twelve times you actually generate repeating choice tasks. The design is still orthogonal, but this is not what you want.

To perhaps understand what happens when generating such a design, Ngene first tries to locate an orthogonal design with 144 rows. This is a very large design with many columns. Then Ngene selects only 8 columns from this orthogonal array, since you only have 8 attributes. However, the design is meant for up to 92 (!) attributes, see the orthogonal array below that Ngene will use. This is the only orthogonal array that we are aware of in 144 rows that has at least 8 columns with 3 levels. When selecting only 8 columns out of these 92 columns, it will result in many repeating rows, as the orthogonal array is designed to estimate a much larger number of variables.

In your case, you could use the design with 12 rows, or use the ;foldover option to get a design with 24 rows in 2 blocks. You can try 36 rows etc, but it will be harder to prevent repeating choice tasks, as the orthogonal array will get larger and larger.

2^76 3^12 6^4 8^1 n=144
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