A few examples of completely randomised experiments are provided below
Abrasion resistance of fabrics
In the manufacture of clothing, a wear-testing machine is used to measure the resistance to abrasion of different fabrics by measuring the weight loss after a fixed number of abrasion cycles. An experiment was conducted to compare four competing fabrics and the results are shown below:
Fabric | |||
---|---|---|---|
A | B | C | D |
1.93 2.38 2.20 2.25 |
2.55 2.72 2.75 2.70 |
2.24 2.68 2.31 2.28 |
2.33 2.40 2.28 2.25 |
If the samples of fabric A had been tested first, the testing machine could have altered as it warmed up, making the abrasion loss for fabric A seem lower than it should have been.
Hardwood and paper strength
A manufacturer of paper used for making grocery bags is interested in improving the tensile strength of the product. Product engineering thinks that tensile strength is a function of the hardwood concentration in the pulp and that the range of hardwood concentrations of practical interest is between 5 and 20%. A team of engineers responsible for the study decides to investigate four levels of hardwood concentration: 5%, 10%, 15% and 20%. They decide to make up six test specimens at each concentration level using a pilot plant. All 24 specimens are tested on a laboratory tensile tester in random order.
Hardwood | Observations | |||||
---|---|---|---|---|---|---|
Concentration (%) | 1 | 2 | 3 | 4 | 5 | 6 |
5 10 15 20 |
7 12 14 19 |
8 17 18 25 |
15 13 19 22 |
11 18 17 23 |
9 19 16 18 |
10 15 18 20 |
If the samples had been tested in order, starting with the six with 5% hardwood, the operator or testing machine could have altered during the course of the experiment, biasing the results.
Quality of silk
A measure of the quality of reeled silk is its "denier" — a number expressing the weight of a fixed length. Among many factors that affect denier, the temperature of the bath in which the cocoons are cooked is likely to be important. An experiment was conducted with two different bath temperatures, 105°F and 110°F. The table below shows the deniers in milligrams of 20 samples of silk at 105°F and 18 samples at 110°F.
Temperature of bath (°F) |
Individual denier values | |||||||||
---|---|---|---|---|---|---|---|---|---|---|
105 | 250 225 |
229 205 |
206 228 |
220 249 |
226 237 |
255 206 |
258 214 |
243 236 |
222 225 |
221 243 |
110 | 235 230 |
258 245 |
225 315 |
207 250 |
260 247 |
268 254 |
256 251 |
220 225 |
224 |
258 |
If the researcher had subjectively picked 20 out of the 38 cocoons for the 105°F bath, larger ones might have been subconsciously picked, biasing the results.