"Jim Snow" <[EMAIL PROTECTED]> wrote in message news:<[EMAIL PROTECTED]>... > "Euh" <[EMAIL PROTECTED]> wrote in message > news:[EMAIL PROTECTED] > > Hello all, > > > > I'm trying to evaluate the concentration of suspended particles �n a > > flask. > > I took 3 differents samples from the flask and, for each sample, I did > > 20 counts under the microscope (= at 20 different locations on the > > microscope slide) > > (snip) > > > > Data (each column is a sample, each row is a count) > > > > 98 108 115 > > 123 102 120 > > 88 90 93 > > 65 71 92 > > 95 56 131 > > 68 145 138 > > 114 136 116 > > 82 100 98 > > 87 85 70 > > 109 116 56 > > 134 157 34 > > 102 60 113 > > 130 90 125 > > 53 121 63 > > 124 111 81 > > 114 92 117 > > 118 131 109 > > 76 55 113 > > 97 256 108 > > 79 146 72 > > Your counts ,I think, are supposed to be counts of the number of > particles in equal volumes of a well mixed suspension. If this were > true, the numbers would follow a Poisson distribution, characterised > by the population variance being equal to the mean. This is clearly > not true for your samples: > > Means are 97.8, 111.4, 98.2 Variances are 527, 2061, 760 > > Either the volumes contributing to each count are variable or mixing > has not been thorough enough, or I do not understand your description > of the data. > > HTH Jim Snow
It's not a problem of mixing, but it's related. Particles tend to form aggregates which makes it harder to count the total number. On top of that, some particles are degraded over time and you end up with debris in the solution. Sometimes, it's hard to tell if what you see is a small particle (that should be counted) or a debris (should be ignored). it's judgment based...hence the huge variation. By adding one count at a time and computing a t-value, I've been able to evaluate that approx 10 counts are required to get close to the true value (15 % error). I've also computed the number of samples required based on the standard deviation of all the counts and this gives me a higher number. . . ================================================================= Instructions for joining and leaving this list, remarks about the problem of INAPPROPRIATE MESSAGES, and archives are available at: . http://jse.stat.ncsu.edu/ . =================================================================
