14 
BULLETIN 563, U. S. DEPARTMENT OF AGRICULTURE. 
Table VII. — Comparison of bacterial counts obtained from the same samples of ice 
cream but with different dilutions — Continued. 
Lot. 
Sam- 
ple 
No. 
Dilu- 
tion. 
Number of 
colonies on 
duplicate 
plates. 
Average 
count per 
c. c. 
Varia- 
tion be- 
tween 
lowest 
and 
highest 
counts. 
Dilution. 
Num- 
ber of 
colonies 
on du- 
plicate 
plates. 
Avcerage 
count per 
c. c. 
Varia- 
tion be- 
tween 
lowest 
and 
highest 
counts. 
D ... 
1 
2 
3 
4 
5 
6 
7 
8 
9 
321 
270 
386 
278 
367 
354 
431 
470 
434 
310 
305 
372 
305 
356 
357 
466 
31,500,000 
28,7.50,000 
37,900,000 
29,150,000 
36,150,000 
35,550,000 
44,850,000 
47,000,000 
43,400,000 
Perct. 
63.47 
l.ooo.iTTo" 
22 
35 
24 
33 
35 
35 
43 
52 
55 
31 
36 
37 
25 
34 
'27' 
39 
45 
26,500,000 
35,500,000 
30,500,000 
29,000,000 
34,500,000 
35,000,000 
35,000,000 
45,500,000 
50,000,000 
Perct. 
88.69 
E.... 
1 
2 
3 
4 
5 
6 
8 
9 
t;S»6 
219 
214 
222 
192 
212 
197 
212 
231 
218 
186 
""ijjo" 
i94* 
200 
208 
205 
224 
202,500 
214,000 
206,000 
192,000 
203,000 
198,500 
210,000 
218,000 
221,000 
15. 10 
TtS.WBTS 
24 
25 
23 
13 
26 
36 
20 
31 
36 
20 
'24' 
22 
16 
'24' 
25 
22 
220,000 
250,000 
235,000 
175,000 
210,000 
360,000 
220,000 
280,000 
290,000 
105. 71 
F .... 
1 
2 
3 
4 
5 
6 
7 
8 
10<&600 
310 
286 
330 
293 
284 
328 
286 
275 
298 
"288" 
288 
292 
278 
285 
30,400.000 
28,600,000 
33.000,000 
29; 050, 000 
28,600,000 
31,000,000 
28, 200, 000 
28,000,000 
17.85 
1.000.017 
19 
27 
21 
29 
31 
36 
35 
27 
13 
29 
22 
22 
28 
31 
39 
32 
16,000,000 
28,000,000 
21,500,000 
25,500,000 
29,500,000 
33,500,000 
37.000,000 
29,500,000 
131.25 
G.... 
1 
2 
3 
4 
5 
6 
7 
8 
9 
j 
144 
128 
179 
200 
235 
152 
130 
270 
222 
178 
136 
161 
"i90' 
"i33' 
i:ooo;ooo 
22 
17 
7 
30 
6 
15 
15 
38 
33 
20 
47 
14 
14 
1 
15 
9 
'45' 
21,000.000 
32,000,000 
10,500,000 
22,000,000 
3,500,000 
15.000,000 
12,000,000 
38, 000; 000 
39,000,000 
1,014.28 
13,200,000 
17,000,000 
20,000.000 
21,250,000 
15.200,000 
13,150.000 
27,000,000 
22,200,000 
VARIATION BETWEEN DUPLICATE COUNTS FROM SAME SAMPLE AND 
SAME DILUTION. 
As stated, we do not intend to say just what variation should be 
allowed between duplicate counts or a series of samples of ice cream 
from the same lot. Table VIII, however, shows some colony counts 
obtained by making a series of from five to eight plates from the 
same dilution. In five different samples a variation of from 7 to 
26.6 per cent was found. Among the duplicate plates in the exami- 
nation of other samples of ice cream a variation as high as 41 per 
cent was found. From figures given by other investigators of the 
results of bacterial counts of ice cream, variations in counts between 
duplicate plates as high as 88 per cent have been observed. It is 
obvious that the factor of variation among plates from the same 
dilution of ice cream must be taken into consideration when inter- 
preting the results of the bacterial analysis of duplicate samples or 
a series of samples from a given mass of ice cream. To this factor 
of variation must be also added that caused by the difficulty of 
removing exactly equal quantities of different samples of ice cream 
in order to make the first dilution. 
