Binomial Distribution Formula


For HP Programmable Calculators

For those with HP programmable calculators (I have the 48GX), this program will save you the trouble of using the funky formula that calculates the probability of a certain count, given a number of trials and probability of event (binomial distribution). NOTE: This program does not calculate the cumulative distribution, just the "joint". As in, it will tell you the probability of an event happening 4 times out of 10 trials if the p=0.5, it will NOT tell you the probability of less than or equal to 4. To do that, you must run the calculation with x=0,1,2,3,4.

Entering the program

  1. Type in the following program and press enter (NOTE: there are special keys for the right arrow ("->"), and for the "<<>>" brackets)
    << -> C X P << X C COMB P C ^ 1 P - X C - ^ * * >>
  2. Press enter to push it onto the stack.
  3. Type "BDIST" and push it onto the stack.
  4. Hit the "STO" key (store)
  5. Hit the "VAR" key (shows variables). You should see the "BDIST" program in the list.

Running the program

To run, simply push the count, number of trials, and probability onto the stack as three separate lines, then press the "BDIST" button. For instance, to calculate the probability of 4 heads out of 10 trials, the keystrokes would be:
  1. 4 - enter
  2. 10 - enter
  3. .5 - enter
  4. "BDIST" key
The result should be: 0.205078125
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