This is a straightforward post of dice tables for 4th edition Shadowrun. A player rolls a number of d6s equal to the sum of the character's relevant attribute and skill. Dice that come up 5 or 6 count as successes. If at least half of the dice come up 1, a Glitch occurs (something bad) even if there are also successes. A Glitch with no successes is a Critical Glitch (something very bad). These tables assume that players do not spend edge. The first table shows the probability of each outcome for up to 15 dice.
What I find particularly interesting here is that the likelihood of both types of Glitch actually increases from 1 die to 2 dice, and that pattern continues for each even number of dice. There is a kind of mechanical penalty for being better at a task half of the time, though probabilities of successes always increases. This bizarre feature of the Glitch system does not make any sense from a simulation or game balance perspective.
This table may help game masters determine task difficulties based on characters' dice totals and the probabilities of success the GM wants in each situation.