Today I’m going to write about a decision that’s made by many American families each year: how to pick summer camps for our kids. There are several issues to take into account, such as cost, benefit, hours, and kids’ preferences. I’ll introduce an optimization model for summer camp selection through a numerical example. The example portrays a large family, but the same ideas apply if a few smaller families want to get together and solve this problem. This way they can take advantage of the discounts and take turns driving the kids around.

The Joneses have six kids: Amy, Beth, Cathy, David, Earl and Fred (yes, their first names are alphabetically sorted, matching their increasing order of age; Mr. and Mrs. Jones always knew they’d have six kids and hence named their firstborn with an ‘F’ name). This year, they’ve narrowed down their list of potential summer camps to the following ten: Math, Chess, Nature, Crafts, Cooking, Gymnastics, Soccer, Tennis, Diving, and Fishing. The Nature camp takes kids on a hike through the woods with the guidance of a biologist; they make frequent stops upon encountering specific plants and animals, during which a mini science lecture is delivered (pretty cool!). The Cooking camp involves cooking chemistry instruction, à la Alton Brown (also pretty cool).

The following table contains some data related to each camp:

The Cost column indicates the cost per child. The Discount column indicates the percentage discount that each child enrolled after the first would receive on the cost of each camp. For instance, if three children are enrolled in Math camp, the first would cost $1100, and the second and third would cost $770 each (30% less). The Hours column is self-explanatory and the last two columns indicate whether or not that particular camp develops mental and physical abilities, respectively (a value of one = yes, zero=no).

The next table shows some of the child-specific requirements:

For example, the Joneses want Fred to attend at least 3 camps that develop mental abilities, and at least 1 camp that develops physical abilities. The last two columns in the above table indicate the minimum and maximum number of camp hours for each child over the 9-week summer break.

The next thing parents need to take into account are their children’s preferences. So here they are:

The smaller the number in the above table, the more desirable a particular camp is. For example, Amy is a bit of a math nerd, and if we were to flip David’s preference scores for Math and Tennis, he could be classified as a bit of a jock. Some conflicts exist, in the sense that not all camps are compatible with each other in terms of time schedules. In this particular case, let’s assume that no child can attend *both* the Soccer and Tennis camps, or *both* the Nature and Soccer camps. Here’s how we are going to use this preference table to create a sense of fairness among the children: whenever a child that prefers camp X to camp Y goes to camp Y and doesn’t go to camp X, nobody else gets to go to camp X either. For example, if Amy goes to Nature camp and isn’t sent to either Math or Chess camp, none of her siblings are allowed to go to Math or Chess either. Conversely, if the Joneses decide to send Earl to Chess camp and Fred to Tennis camp, they must also send Earl to Tennis camp (because Earl prefers Tennis to Chess, and “Fred is going! Why can’t I go too!”). Clearly, there are other ways to use/interpret this table, such as trying to send everyone to at least one of their top N choices, but we won’t consider those alternatives here.

After taking all of the above issues and conditions into account, here’s a solution that satisfies all the requirements while resulting in the minimum cost of **$22,180.00** (You guessed it…the Joneses are probably *not* among the 99%):

**Amy** goes to Math, Crafts, Cooking, and Tennis; **Beth** goes to Math, Cooking, Tennis, and Fishing; **Cathy** goes to Math, Crafts, Tennis, and Fishing; **David** goes to Math, Cooking, and Fishing; **Earl** goes to Math, Tennis, and Fishing; and **Fred** goes to Math, Crafts, Cooking, and Tennis. Mmm…interestingly, everyone goes to Math camp. I think the Joneses are on to something…

Depending on your own requirements, preferences, and costs your solution may differ, of course. But this should give you an idea of how this simple problem can easily become very complicated to solve. No need to fear, though! *Operations Research* is here!

**Food for Thought:** Here’s an interesting question that helps illustrate how high-quality solutions can be counterintuitive: by looking at the preferences table, we see that everyone prefers Soccer to Tennis. In addition, Soccer camp is less expensive than Tennis camp. So how come we send almost everyone to Tennis camp? Isn’t that strange? Let me know what you think in the comments below! That’s one of the advantages of using an analytical approach to decision making: it helps us find solutions we wouldn’t even consider otherwise because they don’t seem to make sense (at least not at first).

If you’re curious about how I managed to find the optimal solution, read on!

**Details of the Analysis:**

To find the minimum-cost solution, we can create a mathematical representation of the problem, a.k.a. a *model*, and then solve this model with the help of a computer. Let’s see how.

The first obvious decision to make is who goes where. So let the binary variable equal when child goes to camp , and equal to otherwise. We’ll also need another binary variable that is equal to when at least one child goes to camp and equal to when none of the children go to camp . We are now ready to write our objective function and constraints. I’ll refer to the problem data using the column headings of the tables above. The subscript will always refer to a child, and the subscript will always refer to a camp.

To minimize the total cost, we write the following objective function:

Note how we are using the variable to handle the discount for sending more than one child to camp : we charge every child the discounted price in the double summation and add the discount back in only once if .

Now we have to deal with the four requirements: minimum and maximum hours, mental activity, and physical activity. For every child , we have to write the following four constraints:

Next, we enforce the preference rules. Let’s recall the example involving Earl and Fred: if Earl goes to Chess camp and someone else (it doesn’t matter who) goes to Tennis camp, then Earl has to go to Tennis camp as well. Here’s what this constraint would look like:

Of course, we have to repeat this constraint for every child and every pair of camps and such that child prefers to in the following way:

The camp compatibility constraints say that no child can attend both Soccer and Tennis, or both Nature and Soccer, therefore:

Finally, we need to relate the and variables by stating that unless , no can be equal to . So we write the following constraint for all values of and :

And that’s the end of our model. Here’s a representation of this mathematical model in AMPL in case you want to play with it yourself. This is the model I used to obtain the numerical results reported above. Enjoy!

This is great! I love your detailed model and analysis. How you enforce the preference rules seems to have a large impact on the results. All of the kids go to math, tennis, and fishing. This is largely because math and fishing have “champions” who prefer these activities to all the rest. So the math-hating children are forced to go to math camp to save a few bucks.

I’m not sure why soccer isn’t selected. I’m dissecting your preference constraints. It seems like if a child goes to one of his least preferred choices (Earl goes to fishing because David loves to fish), then Tennis is a good choice *relative to* the other activities selected for the kids. I don’t have enough time to give you my best answer. Hopefully, some of your other readers can comment on this.

Thanks, Laura! Indeed, the list of preferences and the preference enforcement rules make a big difference. In fact, they explain the counter-intuitive answer. You’re “warm” but not quite there yet :-) If nobody gets it right, I’ll write another post with the explanation.

Tallys,

Nice post! Did you use this model to solve an actual problem?

In the solution returned by the solver, it seems we can’t simply exchange Tennis by Soccer because of David. He was the only one who did not take Tennis. But since he took Cook, which is his number 6 preference, nobody can’t take Soccer because it is his 4 preference.

Have you tested which would be the result if you added an additional constraint requiring at least one child to take soccer?

Guilherme: you’re correct about David. If anyone were sent to soccer camp, David would have to go to soccer camp as well (because of the preference rules). As a consequence, the savings obtained from changing the tennis camps of the other kids to soccer camps would be more than offset by the extra expense of sending David to soccer camp. I didn’t run the test you propose, though. I haven’t used this model on an actual problem either, but it would be nice if someone did! Thank you for your comment and feliz natal!

Pingback: Promote O.R. by Taking Folks by Surprise | O.R. by the Beach