Sunday, May 31, 2020

A Simple Constrained Optimization

A question posted to OR Stack Exchange, "Linear optimization problem with user-defined cost function", caught my eye. The question has gone through multiple edits, and the title is a bit misleading, in that the objective function is in at least some cases nonlinear. The constraints are both linear and very simple. The user is looking for weights to assign to $n$ vectors, and the weights $x_i$ satisfy $$\sum_{i=1}^n x_i = 1\\x \ge 0.$$ Emma, the original poster, put a working example (in Python) on GitHub. The simplified version of her cost function includes division of one linear expression by another, with an adjustment to deal with division by zero errors (converting the resulting NaN to 0).

The feasible region of the problem is a simplex, which triggered a memory of the Nelder-Mead algorithm (which was known as the "Nelder-Mead simplex algorithm" when I learned it, despite confusion with Dantzig's simplex algorithm for linear programs). The Nelder-Mead algorithm, published in 1965, attempts to optimize a nonlinear function (with no guarantee of convergence to the optimum in general), using only function evaluations (no derivatives). It is based on an earlier algorithm (by Spendley, Hext and Himsworth, in 1962), and I'm pretty sure there have been tweaks to Nelder-Mead over the subsequent years.

The Nelder-Mead algorithm is designed for unconstrained problems. That said, my somewhat fuzzy recollection was that Nelder-Mead starts with a simplex (hopefully containing an optimal solution) and progressively shrinks the uncertainty region, each time getting a simplex that is a subset of the previous simplex. So if we start with the unit simplex $\lbrace (1,0,0,\dots,0,0), (0,1,0,\dots,0,0),\dots,(0,0,0,\dots,0,1)\rbrace$, which is the full feasible region, every subsequent simplex should be comprised of feasible points. It turns out I was not quite right. Depending on the parameter values you use, there is one step (expansion) that can leave the current simplex and thus possibly violate the sign restrictions. That's easily fixed, though, by checking the step size and shrinking it if necessary.

There are several R packages containing a Nelder-Mead function, but most of them look like they are designed for univariate optimization, and the one I could find that was multivariate and allowed specification of the initial simplex would not work for me. So I coded my own, based on the Wikipedia page, which was easy enough. I used what that page describes as typical values for the four step size parameters. It hit my convergent limit (too small a change in the simplex) after 29 iterations, producing a solution that appears to be not quite optimal but close.

Just for comparison purposes, I thought I would try a genetic algorithm (GA). GAs are generally not designed for constrained problems, although there are exceptions. (Search "random key genetic algorithm" to find one.) That's easy to finesse in our case. Getting a GA to produce only nonnegative values is easy: you just have to require the code that generates new solutions (used to seed the initial population, and possibly for immigration) and the code that mutates existing solutions to use only nonnegative numbers. That might actually be the default in a lot of GA libraries. "Crossover" (their term for solutions having children) takes care of itself. So we just need to enforce the lone equation constraint, which we can do by redefining the objective function. We allow the GA to produce solutions without regard to the sum of their components, and instead optimize the function $$\hat{f}(x)=f\left(\frac{x}{\sum_{i=1}^n x_i}\right)$$where $f()$ is the original objective function.

R has multiple GA packages. I used the `genalg` package in my experiments. Running was 100 generations with a population of size 200 took several seconds (so longer than what Nelder-Mead took), but it produced a somewhat better solution. Since the GA is a random algorithm, running it repeatedly will produce different results, some worse, possibly some better. You could also try restarting Nelder-Mead when the polytope gets too small, starting from a new polytope centered around the current optimum, which might possibly improve on the solution obtained.

This was all mostly just to satisfy my curiosity. My R code for both the Nelder-Mead and GA approaches is in an R notebook you are free to download.

Saturday, May 23, 2020

Of ICUs and Simulations

I'm a fan of the INFORMS "Resoundingly Human" podcasts, particularly since they changed the format to shorter (~15 minute) installments. I just listened to a longer entry (40+ minutes) about the use of OR (and specifically simulation models) to help with hospital planning during the pandemic. (Grrrr. I'd hoped to keep the word "pandemic" out of my blog. Oh well.) The title is "The dangers of overcrowding: Helping ICUs preserve essential bed space", and the guest is Frances Sneddon, CTO of Simul8 Corporation. I thought the content was interesting, and Frances was very enthusiastic presenting it, so if you have any interest in simulation and/or how OR can help during the (here it comes again) pandemic, I do recommend giving it a listen.

One thing that definitely got my attention was Frances's emphasis on building simulation models in a rapid / interactive / iterative / agile way. ("Rapid" was very much her word, and she used "agile" toward the end of the podcast. "Interactive" and "iterative" are my words for the process she described.) Basically (again with my paraphrasing), she said that the best outcomes occur when simulations are born from discussions among users and modelers where the users ask questions, followed by fairly rapid building and running of a new/revised model, followed by discussions with the users and more of the same. Frances at one point drew an analogy to detective work, where running one simulation lets you ferret out clues that lead to questions that lead to the next model.

To some extent, I think the same likely holds true of other applications of OR in the real world, including optimization. Having one conversation with the end users, wandering off into a cave to build a model, and then presenting it as a fait accompli  is probably not a good way to build trust in the model results and may well leave the user with a model that fundamentally does not get the job done. As a small example, I once worked on a model for assigning school-age children to recreational league athletic teams. The version of the model satisfied all stated constraints, but the user told me it would not work. Some parents have multiple children enrolled in the league, and it is unworkable to expect them to ferry their kids to different teams playing or practicing in different places. So siblings must go on the same team. (There were other constraints that emerged after the initial specification, but I won't bore you with the details.)

So on the one hand, I'm predisposed to agree at least somewhat with what Frances said. Here comes the cognitive dissonance (as my erstwhile management colleagues would say). Once upon a time I actually taught simulation modeling. (I won't say exactly when, but in the podcast Frances mentions having been in the OR field for 20 years, and how saying that makes her feel old. The last time I taught simulation was before she entered the field.) Two significant issues, at least back then, were verifying and validating simulation models. I suspect that verification (essentially checking the correctness of the code, given the model) is a lot easier now, particularly if you are using GUI-based model design tools, where the coding looks a lot like drawing a flow chart from a palette of blocks. The model likely was also presented as a flow chart, so comparing code to model should be straightforward (put the two flow charts side by side). Validation, the process of confirming that the model is correct, may or may not be easier than in the past. To some extent you can achieve "face validity" by talking through the assumptions of the model with the users during those interactive sessions, helped by a flow chart.

Back in my day, we also talked about historical validation (running the model with historical inputs and seeing if the results reasonably tracked with historical outputs). When you are trying to answer "what if" questions (what if we reconfigure the ICU this way, or change admissions this way, or ...?), you likely don't have historical data for the alternate configurations, but you can at least validate that the model adequately captures the "base case", whatever that is. Also, "what if" questions are likely to lead you down paths for which you lack hard data for parameter estimates. What if we build a tent hospital in Central Park (which has never been done before)? What do we use for the rate at which patients experience allergy attacks (from plant life in the park that simply does not exist inside the hospital)? My guess is that your only recourse is to run the simulation for multiple values of the mystery parameter, which leads us to a geometric explosion of scenarios as we pile on uncertain parameters. So my question is this: in an interactive loop (meet with users - hack model - collect runs / synthesize output - repeat), can we take reasonable care to preserve validity without exhausting the parties involved, or overloading them with possibilities to the point that there is no actual take-away?

Informed opinions are welcome in the comments section. (It's an election year here, so I'm already maxed out on uninformed opinions.)