4i 8 Stiles and j 0 rgensen. — Studies in Permeability. V. 
that the probable errors (P M ) of the mean results there recorded lie 
between 2 and 4 per cent. 
With carrot the variability of the tissue is somewhat greater, as the 
following table shows : 
Table II. 
Water absorbed by 20 grin, of Carrot from Running Tap-water. 
Sample. 
Increase in 
Weight of 20 grm. 
of Carrot after various times. 
21-03 hrs. 
44*4 hrs. 
68-8 hrs. 
123-5 hri 
grm. 
grm. 
grm. 
grm. 
1 
2*53 
2-41 
2-60 
2-48 
2 
2-52 
2*20 
2*54 
2*57 
3 
2-24 
2-37 
2-74 
2-69 
4 
2-14 
2*26 
2*56 
2-46 
5 
3*09 
2'83 
3-00 
3 ’°° 
6 
3.21 
3 - 3 ^ 
3-66 
3-66 
7 
2*79 
3*oo 
3*57 
3 ‘ 6 i 
8 
3-io 
3*34 
3-25 
3- 2 2 
9 
2-66 
3-00 
2-74 
2-87 
Mean 
2*70 
2-75 
2*96 
2*95 
Pm 
+ 0-08 
+ 0*10 
+ 0*07 
+ 0-10 
It is thus possible when working with nine samples to obtain results in 
which the probable error of the mean result of the nine samples is about 
3 per cent. Thus differences of about 9 per cent, have a reasonably certain 
significance in relation to different experimental conditions. This is not 
a particularly high order of accuracy having regard to the labour entailed 
when sets of nine different samples are used. We therefore adopted the 
following method in order to reduce the sampling error : 
All the discs required for a particular experiment in which the results 
were to be compared were cut, dried with blotting-paper, and then 
thoroughly mixed before they were weighed out into lots of 10 or 20 grm. 
This preliminary mixing has the result of reducing the sampling error con- 
siderably, as the following table shows. Thus when carrot is used with only 
four different samples the probable error of the mean does not generally rise 
much above 2 per cent. With potato it is possible to work with sets of 
three samples and obtain this same degree of accuracy. 
Table III. 
Water absorbed by 20 grm. of Carrot from Distilled Water. 
Sample . Increase in Weight of 20 grm. of Carrot after various tunes. 
o*5 hr. 
1 <o hr. 
2.0 hrs. 
5-0 hrs. 
22*5 hrs. 
49'5 hr 
grm. 
grm. 
grm. 
grm. 
grm. 
grm. 
I 
I *43 
T -95 
2 ‘75 
3' T 7 
3 ‘ 4 8 
3‘65 
2 
1-50 
2-1 1 
3.10 
3-45 
3‘ 8 7 
4-00 
3 
i -47 
2*02 
2-85 
3.10 
3 * 4 ° 
3*75 
4 
J *47 
1 ’94 
2<74 
2-92 
3*19 
3-62 
Mean 
1 ‘47 
2-00 
2-86 
3 *i 6 
3‘47 
3*75 
Pm 
±°»oi 
±0-03 
+ o-o6 
±0.07 
+ 0*10 
+ o-o6 
