A primer on reading field research results
MSU Extension educators offer farmers data to consider from field research trials. Here’s some background on understanding the statistics side of the data.
Field research results are often shared by consultants, salesmen, and MSU Extension educators to assist farmers in determining if a particular practice or product is worth adopting.
Statistics provide a tool to determine the value and reliability of the data provided. Two common statistical values used in presenting research data are LSD 0.05 and CV. In addition to the use of these values, data can be presented in manner that allows the viewer a quick statistical evaluation.
That’s the Least Significant Difference at the 0.05 level. The LSD represents minimum difference between treatments in the study (whether it’s a fungicide treatment, difference in population, difference in fertilizer rate, etc.) for the difference to be attributed to the treatment rather than some external factor like soil variability, difference in drainage, insect feeding, etc.
An LSD at the 0.05 level means that 95 percent of the time, given similar circumstances, I should get similar or repeated results. Less frequently, researchers will report the LSD 0.10, or that 90 percent of the time, given similar circumstances, I should get similar or repeated results.
That stands for Coefficient of Variation. The CV is a measure of the variability of the data that cannot be explained by the statistics. Again, there are factors other than the treatment that can influence the data, like number of replications, weed pressure, soil variability and rainfall. The lower the CV, the more confident you can be that the study is good. In field-based research, a CV of less than 10 percent is usually considered good data, and a CV of less than 5 percent is usually considered very good data. Researchers find sites with uniform soils, keep weed pressure minimized, replicate treatments three of four times, and do other things to minimize variation, but some variation is inherent to field research.
Presentation of data
When showing that one or more treatments were statistically different from other treatments, researchers usually choose one of two presentation formats. The first is to use letters like a, b, c, d, etc. to show significant differences. See Table 1.
Table 1. Corn Response to Sidedressed Nitrogen
|Sidedress N (lbs./A)||Moisture (%)||Yield (bu./A)|
|LSD @ 0.05||15|
In this trial, we are 95 percent certain that the two treatments with the lowest sidedressed N (0 and 40 pounds) yielded significantly lower than any treatment that yielded greater than 123 bushels (108 + 15 (the LSD) = 123). They are designated with a “c”, or they are the lowest yielding treatments. The 0 and 40 pound treatments also yielded statistically similarly to the 80 pound treatment, because they were within 15 bushels of the 80 pound treatment. The 80 pound treatment also yielded within 15 bushels of the 160 pound treatment. But the 160 pound treatment yielded more than 15 more than the 0 and 40 pound treatments, so the 80 pound treatment is designated with a “bc”, showing that it was within 15 bushels of both the lowest yielding and second lowest yielding treatments. The 160 pound treatment yielded within 15 bushels of the 80 pound treatment and the 120, 200 and 240 pound treatments, so it is designated with an “ab”. The 120, 200 and 240 pound treatments, yielded more than 15 bushels greater than anything designated with a “c” or “b”, so they are designated with only an “a”, or they are the highest yielding treatments.
A second way that researchers present data is to bold the treatments that yielded statistically greater than other treatments. See Table 2.
Table 2. Corn Hybrid Trial – Refuge (no Bt)
|Hybrid||Moisture (%)||Yield (bu.)|
|Great Lakes 4689RR||19.0||179.3|
|Great Lakes 4481RR||17.1||173.6|
|Masters Choice MCT-480||18.1||167.8|
|LSD @ 0.05||5.5|
In this case, the researchers wanted to show statistically significant differences from the top yielding hybrid. The top yielding hybrid yielded 184.2 bushels per acre. The LSD at the 0.05 level was 5.5 bushels. Therefore, any hybrid that yielded within 5.5 bushels of 184.2, or 178.7 bushels or greater, yielded statistically the same as the top yielding hybrid. Only one other hybrid yielded statistically the same as the top yielding hybrid. The other seven hybrids all yielded statistically less than the top yielding hybrid.