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Re^2: standard deviation accuracy question

by tlm (Prior)
on Jul 22, 2005 at 15:56 UTC ( [id://477268]=note: print w/replies, xml ) Need Help??


in reply to Re: standard deviation accuracy question
in thread standard deviation accuracy question

I agree with Excel, for the most part:

condition1 50.73 condition2 44.34 condition3 52.05 condition4 56.39

use strict; use warnings; use List::Util 'sum'; sub sample_sd { die unless @_ and ref $_[ 0 ] eq 'ARRAY' and @{ $_[ 0 ] } > 1; my $data = shift; my $sum = sum @$data; my $n = @$data; my $mean = $sum/$n; my $sq_dev = sum map $_*$_, map $_ - $mean, @$data; return sqrt( ( $sq_dev )/( $n - 1 ) ); } while ( <DATA> ) { my @data = split; my $label = shift @data; my $sd = sample_sd( \@data ); printf "$label %.2f\n", $sd; } __DATA__ condition1 397 322 350 346 338 324 461 477 432 373 386 412 475 360.5 3 +84 324 424 319 454.5 412 387 370 condition2 360.5 318.5 356 316.5 330 296.5 444 474 416.5 355 320 401.5 + 363.5 332 363 324 388 312 344 372 347.5 338 condition3 372 354 358 311 336.5 297 485 461 407.5 365 342 428 494 343 +.5 372 324 379.5 345 389 384 363 346 condition4 324 305.5 364 320 327.5 288.5 432.5 434 447 369 315 397.5 5 +15 339 405 330 396 319 345 381.5 340 317

Here's a more efficient alternative, closer in spirit to the algorithm you posted originally, but properly making the finite-sample correction:

use List::Util 'sum'; sub sample_sd { die unless @_ and ref $_[ 0 ] eq 'ARRAY' and @{ $_[ 0 ] } > 1; my $data = shift; my $sum = sum @$data; my $sum2 = sum map $_*$_, @$data; my $n = @$data; return sqrt( ( $sum2 - (( $sum*$sum )/$n) )/($n-1) ); }

the lowliest monk

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