HP 33s - Manual (Page 154)

11–10 Statistical Operations
Normalizing Close, Large Numbers
The calculator might be unable to correctly calculate the standard deviation and
linear regression for a variable whose data values differ by a relatively small
amount. To avoid this, normalize the data by entering each value as the difference
from one central value (such as the mean). For normalized x–values, this difference
must then be added back to the calculation of
x
and xˆ , and
yˆ
and b must
also be adjusted. For example, if your x–values were 7776999, 7777000, and
7777001, you should enter the data as –1, 0, and 1; then add 7777000 back to
x
and xˆ . For b, add back 7777000 × m. To calculate
yˆ
, be sure to supply
an x–value that is less 7777000.
Similar inaccuracies can result if your x and y values have greatly different
magnitudes. Again, scaling the data can avoid this problem.
Effect of Deleted Data
Executing {  does not delete any rounding errors that might have been
generated in the statistics registers by the original data values. This difference is
not serious unless the incorrect data have a magnitude that is enormous compared
with the correct data; in such a case, it would be wise to clear and reenter all the
data.
Summation Values and the Statistics Registers
The statistics registers are six unique locations in memory that store the
accumulation of the six summation values.
Summation Statistics
Pressing |  gives you access to the contents of the statistics registers:
„ Press {Q} to recall the number of accumulated data sets.
„ Press {º} to recall the sum of the x–values.
„ Press {¸} to recall the sum of the y–values.
„ Press {;º  }, {;¸  }, and {;º¸} to recall the sums of the squares and the sum
of the products of the x and y — values that are of interest when performing
other statistical calculations in addition to those provided by the calculator.