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human
[Computer Science] How does a computer decide which wireless access point to connect to, assuming all are on the same network?

I'm looking with respect to wireless standard (802.11 b, g, n, ac), proximity, and radio frequency (2.5 or 5 GHz) (not all at once either, assume only the one whose change is being examined is variable). This is assuming all access points are on the same network with the same network name.
gpt
In an ideal world the wireless client associates with the AP with the loudest signal.  A "perfect" signal is around -35dBm (if you're directly next to the AP) and most wireless cards disconnect around -85dBm.  This range  is reduced if lower data rates are disabled; the lower the data rate, the further away you can be, but this reduces the performance of all clients.

This gets tricky when you mix 2.4GHz and 5GHz.  The 5GHz radios operate at half the power of 2.4GHz radios and the signal attenuates (degrades) faster than 2.4GHz, BUT even with a supposedly weaker signal a client can get faster speeds from 802.11a.

Roaming is also up to the client.  If the client can see two APs at -55dBm then it's pretty much a 50/50 chance on which it will pick.  The client will associate to one and only attempt the other if the signal level drops too far.  Some devices will maintain a death grip on an AP before roaming, while others will hop between APs like a jack rabbit.

A lot of this depends on the driver implementation.  For example, iDevices up until iOS 4 or 5 preferred 2.4GHz radios over 5GHz radios even if the 5GHz could give a higher data rate.  After that point the developers swapped the preference.  The same thing happened with Dell Inspiron wireless drivers.  The drivers also determine how the signal strength is calculated.

EDIT: Quick guide to frequencies and speeds (physical data rate)

* 802.11b - 2.4GHz - 1, 2, 5.5, 11 (Mbps)
* 802.11g - 2.4GHz - 6, 9, 12, 18, 24, 36, 48, 54 (Mbps)
* 802.11a - 5GHz - 6, 9, 12, 18, 24, 36, 48, 54
* 802.11a/n - 5GHz - 15, 30, 45, 60, 90, 120, 135, 150
* 802.11b/g/n - 2.4GHz - 7.2, 14.4, 21.7, 28.9, 43.3, 57.8, 65, 72.2

Note that "n" works on both 2.4 and 5GHz.  This is why it is important to look for a/n because a lot of consumer devices that advertise 802.11n are actually running at 2.4GHz.