I haven't worked out the precise mathematics of it all, but I'm pretty sure that the metabolism (and therefore screech-producing power) of six-to-seven-year-old girls increases at a rate equal to the square of the number of girls present. There may be an additional variable influence involving exactly how tired the observer is, number of cupcakes consumed, Gwen Stefani, trampolines, and other factors that students at Caltech are doubtless writing theses on at this very moment.
Let me explain:
Having watched my wife raise two girls, and having attended a fair number of birthday parties at which packs of small girls and boys ran around yelling and having a good time, I spent last week not too terribly concerned about my oldest daughter's impending party.
Correction: Impending slumber party. The distinction, as it happens, is quite significant.
First of all, the majority of parties that our kids have attended (or that we have thrown) have generally involved the parents of the other children remaining in attendance. Of course, in the "slumber" variant, the parents generally run off and have a good time. (Now that I think about it, they probably spent a lot of that time laughing at our foolish notions about keeping their kids under control.)
The corollary of that factor, however, is much more important: the children know that their parents aren't around, and aren't going to be around for some time. The practical upshot of this is that the girls, whose running around and shrieking would soon irritate their own parents into action (or at least those parents that I would care to associate with), are suddenly thrust into a sort of power vacuum, allowing for near infinite expansion of their "Shriek Horizon."
While investigating this fascinating phenomenon, I was looking for a graph of noise levels, in order to estimate the volume of the sound in question, and I was startled to find that I was not the first to approach this from a scientific standpoint:
I'm not kidding here. These kids exhibited hyperactivity at a level that I'm pretty sure Einstein proved wasn't possible. And there were only five of them (six if you count my three-year-old, and god help you if you don't.) And they didn't calm down ever.
My wife got her sweet revenge on me for "letting" her plan the party. At 11:30 or so, she popped in some earplugs and went to bed. I, on the other hand, got to get up every five minutes or so and explain the concept of sleep to children who really weren't buying into my pitch, until, finally, I had to sit in a chair and will them to sleep with sheer stubbornness, all the while contemplating the irony of the term "slumber party."