:orphan: .. _ahelp_cdf: *** cdf *** .. contents:: :local: | .. code-block:: none Usage 1: cdf [-v] -t2p statname t params Usage 2: cdf [-v] -p2t statname p params Usage 3: cdf [-v] -t2z statname t params This program does various conversions using the cumulative distribution function (cdf) of certain canonical probability functions. The optional '-v' indicates to be verbose -- this is for debugging purposes, mostly. Use this option if you get results you don't understand! Usage 1: Converts a statistic 't' to a tail probability. Usage 2: Converts a tail probability 'p' to a statistic. Usage 3: Converts a statistic 't' to a N(0,1) value (or z-score) that has the same tail probability. The parameter 'statname' refers to the type of distribution to be used. The numbers in the params list are the auxiliary parameters for the particular distribution. The following table shows the available distribution functions and their parameters: statname Description PARAMETERS -------- ----------- ---------------------------------------- fico Cor SAMPLES FIT-PARAMETERS ORT-PARAMETERS fitt Ttest DEGREES-of-FREEDOM fift Ftest NUMERATOR and DENOMINATOR DEGREES-of-FREEDOM fizt Ztest N/A fict ChiSq DEGREES-of-FREEDOM fibt Beta A (numerator) and B (denominator) fibn Binom NUMBER-of-TRIALS and PROBABILITY-per-TRIAL figt Gamma SHAPE and SCALE fipt Poisson MEAN EXAMPLES: Goal: find p-value for t-statistic of 5.5 with 30 degrees of freedom COMMAND: cdf -t2p fitt 5.5 30 OUTPUT: p = 5.67857e-06 Goal: find F(8,200) threshold that gives a p-value of 0.001 COMMAND: cdf -p2t fift 0.001 8 200 OUTPUT: t = 3.4343 The same functionality is also available in 3dcalc, 1deval, and ccalc, using functions such as 'fift_t2p(t,a,b)'. In particular, if you are scripting, ccalc is probably better to use than cdf, since the output of ccalc -expr 'fitt_t2p(3,20)' is the string '0.007076', while the output of cdf -t2p fitt 3 20 is the string 'p = 0.0070759'.