Dear Statalisters,

`This posting is as much a plea for basic statistical advice as it is for
``assistance with Stata, so I apologize in advance.
`

`I am trying to model the correlates of the timing and severity of
``repeated events, in particular, how a particular type of investment at a
``number of locations responds to local economic conditions over several
``years. Investment occurs in "bursts". At given location there are
``typically intervals of several years with zero investment, followed by
``one year in which investment takes place. The quantity of investment,
``which I seek to model, varies both across locations, and, for the ~10%
``of locations where I investment occurs multiple times (< 4), over years.
``My covariates are measured as panel data, with no censoring.
`

`Is anyone familiar with a statistical model which is appropriate for
``this kind of process? I have looked for clues the medical literature,
``and the closest fit I could find is a survival model of headache
``incidence in which the outcome is classified into ordered categories:
`

`Berridge D M; Whitehead J (1991). Analysis of failure time data with
``ordinal categories of response. Statistics in medicine 10:1703-10.
`

`I am currently awaiting a copy of this article via interlibrary loan,
``but I fear that adapting such a framework to deal with continuous data
``is beyond my competence. Looking closer to home, I could use -heckman-
``or -selmlog- to estimate a linear model with site and year fixed effects
``that controls for selection, but I am unclear whether these methods are
``able to capture the *cumulative* impact of the covariates on the hazard
``of event occurrence over the span of the inter-investment intervals. I
``can also average my covariates over these intervals, convert my dataset
``to spell format, and estimate a survival model, but this only solves
``half the problem. The issue then is how to use the results of the
``survival analysis to account for fact that the covariates jointly
``influence both the selection hazard *and* the magnitude of investment
``once it occurs, e.g., through a quantity like the Inverse Mills Ratio in
``-heckman, twostep-.
`

`Is this actually a simple problem that my own lack of statistical acumen
``is making too complicated? It is hard for me to imagine that someone
``hasn't dealt with a similar question before, and there must be something
``simple that can be done short of developing and estimating an entire
``structural DP model (which in this case is like Rust's (1987) bus engine
``replacement model with engines of different sizes!). Any guidance from
``more experienced and able researchers would be greatly appreciated.
`
Thanks,
-i
--
Ian Sue Wing 675 Commonwealth Ave., Boston MA 02215
Associate Professor Tel: (617) 353-5741
Dept. of Geography & Environment Fax: (617) 353-5986
Boston University Web: http://people.bu.edu/isw
*
* For searches and help try:
* http://www.stata.com/help.cgi?search
* http://www.stata.com/support/statalist/faq
* http://www.ats.ucla.edu/stat/stata/