I had initially decided to not pick more on group-based modelling, but here we go:
In his recent essay on group-based modelling in Journal of Research in Crime and Delinquency (see earlier posts here and here), Nagin discusses two examples of uses of group-based modelling in developmental criminology. It is not clear whether these are mentioned because they are particularly good examples, as they are presented as “early examples”. Maybe it is of historical interest, or these examples are mentioned because they are much cited. Since Nagin’s article is basically promoting GBTM, I assume these are mentioned because they are good examples of what can be achieved using this method. In any case, I would have liked to see examples where GBTM really made a difference.
The first example is the article by Nagin, Farrington and Moffitt who, according to Nagin using SPGM made an important contribution for the following reason:
…what was new from our analysis was the finding that low IQ also distinguished those following the low chronic trajectory from those following the adolescent limited and high chronic trajectories. This finding was made possible by the application of GBTM to identify the latent strata of offending trajectories present in the Cambridge data set.
The article by Nagin, Farrington and Moffitt shows that the relationship between IQ and delinquency varies over groups. (One could of course also say that the relationship is non-linear, but they stick to discussing groups). Which is fine, but the contribution is mainly how the groups are created – although some elaborate testing of equal parameters are involved. If other ways of summarizing the criminal careers (as either continuous or groups) would not find anything similar, it remains what is a methodological artefact and not. However, only one methodological approach was used, so it is hard to assess whether GBTM actually was the only way of discovering this kind of relationship. Maybe alternative methods (e.g. subjective classifications or a continuous index) would have found the exact same thing in these data? Could very well be.
The second example mentioned by Nagin is also written by himself together with Laub and Sampson. This is a very influential paper on the influence of marriage on crime, but has a major flaw because of how GBTM is used. I have recently written an review article together with Savolainen, Aase and Lyngstad on the marriage-crime literature published in Crime and Justice. We commented on this paper as follows:
…they estimated group-based trajectories (Nagin and Land 1993) for the entire observational period from age 7 to 32 and then assigned each person to a group on the basis of posterior probabilities. In the second
stage, they regressed the number of arrests in each 2-year interval from age 17 to 32 on changes in marital status and quality, controlling for group membership and other characteristics. In addition, they conducted separate regression analyses by trajectory group membership.
…
We are somewhat hesitant to endorse this conclusion for methodological reasons. Because the trajectories
were estimated over the entire period—including the marital period—controlling for group membership implies controlling for post-marriage offending outcomes as well. We expect this aspect of the analytic
strategy to bias the results, but further efforts are needed to assess the substantive implications of this methodological approach.
I think our final sentence of this quote very mild. They were partly conditioning on the outcome variable and that is bound to lead to trouble. Frankly, I do not know how to interpret these estimates. In this case, GBTM made a real difference, but to the worse. This was probably hard to see at the time since GBTM had not yet been subject to much methodological scrutiny yet. It is easier to see now.
In sum, although these two studies have other qualities, they are not examples of real success stories of GBTM. My advice would be to come up with some really good examples. But perhaps the only real success story of GBTM is Nagin and Lands 1993-article (for reasons given here).
P.S. Actually, I know of far better examples of using group-based modelling. Neither of them is dependent on GBTM, but it adds a nice touch. For example, Haviland et al use GBTM to improve propensity score matching. Another example is my own work with Jukka Savolainen where offending in the pre-job entry period is summarized using GBTM. For both these studies, other techniques could have been used, but GBTM works very well. There exist also other sound applications.
The post Best practise of group-based modelling appeared on The Grumpy Criminologist 2016-08-11 12:36:42 by Torbjørn.