Читать книгу Social Work Research Methods - Reginald O. York - Страница 59

The fact that I had eggs for breakfast this morning does not necessarily mean that I prefer eggs over cereal for breakfast in general. It could be that I have eggs half the time and cereal half the time and I just happened to have had eggs this morning. If you observed me at breakfast several times and noted that I had eggs each and every time, you would have more reliable evidence that I prefer eggs for breakfast. The more observations you make, the more confident you would be in your conclusion that I prefer eggs for breakfast.

We are referring to a thing called “probability.” Let’s discuss this concept in a general way. Logic would suggest that there is a 50% chance of getting a heads on a given flip of a coin because there are only two possibilities—heads and tails. But let’s suppose that someone said that there was one coin in a set of coins that was rigged to land on heads more often than on tails because of the distribution of the weight of the coin. You pick out one coin, and you want to know if this is the one that is rigged. Let’s suppose that your first flip was heads and the second was also heads. Are you convinced you have the rigged coin? Probably not because you have only flipped it 2 times and we know that two heads in a row can happen just by chance. What if you have flipped this coin 10 times and it came out heads every time? Now you have more reason to believe that you have the rigged coin. A similar result after 20 flips would be even better. If you do not have the rigged coin, you would not likely have very many flips in a row that were similar. The more flips you have that are similar, the better are your chances that you have found the rigged coin. Determining how many flips you need to be confident is a matter for statistics. If you knew how to use a statistical test known as the binomial test, you could see that 5 flips in a row with only heads appearing would be so unusual that you would be safe to bet that you have found the rigged coin.

Now let us put the same lesson to use with a more practical example. Suppose that you wanted to know whether males and females differ in their satisfaction with instruction in research courses. Are females higher or lower than males in their level of satisfaction? You could ask a given group of students if they are generally satisfied with their research instruction, with the options of YES or NO. You could then compare the proportion of females who answered YES with the proportion of males who answered YES. What if you found that 63% of females were satisfied and that 65% of males were satisfied? Does that mean you can conclude that there is truly a difference between males and females? If so, would you be prepared to bet a large sum of money that a new study of this subject would result in males having a higher level of satisfaction? I doubt that you would, because you would realize that this small a difference between males and females could be easily explained by chance. If you had found that 60% of females were satisfied as compared with only 40% of males, you would be more likely to see this difference as noteworthy. However, such a difference with a sample of only 10 students would likely make you wonder if you should take these results seriously. Results with a sample of 100 students would be much more impressive.

You examine the theme of probability in scientific research with the use of statistics. A statistical test applied to your data will tell you the likelihood that these data could have occurred by chance. If you fail to achieve statistical significance with your data, you cannot rule out chance as a likely explanation of them. Thus, you cannot take them seriously in your conclusions. Suppose you found that students had a slightly higher score on knowledge of scientific research at the end of a lesson than before the lesson began but your data failed to be statistically significant. Under these circumstances, you should conclude that you failed to find that your students improved in research knowledge. You should not conclude that they had a slight improvement. Why? Because your data can be explained by chance, and you should not take them seriously. If you had found your data to be statistically significant, then you could conclude that you found that your students had achieved a slight gain in knowledge.