Chapter 5: Analysing the Data |
What -level?One of the key concepts in hypothesis testing is that of significance level (or, equivalently the alpha ( ) level) which specifies the probability level for our evidence to be an unreasonable estimate. By unreasonable, we mean that the estimate should not have taken its particular value unless some non-chance factor(s) had operated to alter the nature of the sample such that it was no longer representative of the population of interest. The researcher has complete control over the value of this significance level. While we recommend a standard, or decision criterion, of = 0.05, ( for two-tailed tests), you should be cautious about the blind adoption of this level. The -level should be considered in light of the research context and in light of your own personal convictions about how strong you want the evidence to be, before you will conclude that a particular estimate is reasonable or unreasonable. In some exploratory contexts (perhaps, in an area where little previous research has been done), you might be willing to be more liberal with your decision criterion and relax the level of significance to 0.10 or even 0.20 - thus, less extreme values of a statistic would be required for you to conclude that non-chance factors had operated to alter the nature of the sample. On the other hand, there are research contexts in which one would want to be more conservative and more certain that an unreasonable estimate had been found. In these cases, the significance level might be lowered to 0.001 (0.1%) where more extreme values of a statistic would be required before non-chance factors were suspected You need to examine the SPSS output usually under headings such as "Sig." or "Two-tailed Sig.", or "Prob." for the probability (or "p-value") of your results being a real difference or a real relationship. This can then be the probability you quote as being the "level of significance" associated with your results. Note that we normally need this p-value to be less than or equal to 0.05 to make the claim of ÔsignificantÕ. It is then up to your discussion to explain/justify/interpret this level of significance to your reader. As a general guide, treat the p-value as a measure of the confidence or faith you can have in your results being real (and not being due to chance fluctuations in sampling). When evaluating your test statistic (Step 4) take into consideration the points raised in the preceding paragraph. The -level is the probability or Ôp-valueÕ you are willing to accept as significant. Ideally, this -level. The -level can also be interpreted as the chance of making a Type I error. |
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