Wherever the observed data doesn't fit the model, the likelihood that the variables are dependent becomes stronger, thus proving the null hypothesis incorrect!
I will not discuss this in more detail, but it is important to know that the null hypothesis is not some abstract "fact" about the test, but rather a choice you make when calculating your model.
Entire books are devoted to the statistical method known as . This section will contain only three paragraphs. This is in part because of the view of some statisticians that ANOVA techniques are somewhat dated or at least redundant with other methods such as (see ). In addition, a casual perusal of the worm literature will uncover relatively scant use of this method. Traditionally, an ANOVA answers the following question: are any of the mean values within a dataset likely to be derived from populations that are truly different? Correspondingly, the null hypothesis for an ANOVA is that all of the samples are derived from populations, whose means are identical and that any difference in their means are due to chance sampling. Thus, an ANOVA will implicitly compare all possible pairwise combinations of samples to each other in its search for differences. Notably, in the case of a positive finding, an ANOVA will not directly indicate which of the populations are different from each other. An ANOVA tells us only that at least one sample is likely to be derived from a population that is different from at least one other population.
Regardless of the method used, the -value derived from a test for differences between proportions will answer the following question: What is the probability that the two experimental samples were derived from the same population? Put another way, the null hypothesis would state that both samples are derived from a single population and that any differences between the sample proportions are due to chance sampling. Much like statistical tests for differences between means, proportions tests can be one- or two-tailed, depending on the nature of the question. For the purpose of most experiments in basic research, however, two-tailed tests are more conservative and tend to be the norm. In addition, analogous to tests with means, one can compare an experimentally derived proportion against a historically accepted standard, although this is rarely done in our field and comes with the possible caveats discussed in . Finally, some software programs will report a 95% CI for the difference between two proportions. In cases where no statistically significant difference is present, the 95% CI for the difference will always include zero.
To narrow the scope of this study, the researcher identified three hypotheses that were used to direct this effort: Hypotheses 1: Young African American women struggle with self-esteem, self-image, depression, abuse, and sexuality; Hypotheses 2: Young African American women struggle with singleness while balancing their careers and family life; and Hypotheses 3: Young African American women have a desire and need for authentic relationships with older, spiritually mature women.
proportions or distributions refer to data sets where outcomes are divided into three or more discrete categories. A common textbook example involves the analysis of genetic crosses where either genotypic or phenotypic results are compared to what would be expected based on Mendel's laws. The standard prescribed statistical procedure in these situations is the test, an approximation method that is analogous to the normal approximation test for binomials. The basic requirements for multinomial tests are similar to those described for binomial tests. Namely, the data must be acquired through random sampling and the outcome of any given trial must be independent of the outcome of other trials. In addition, a minimum of five outcomes is required for each category for the Chi-square goodness-of-fit test to be valid. To run the Chi-square goodness-of-fit test, one can use standard software programs or websites. These will require that you enter the number of expected or control outcomes for each category along with the number of experimental outcomes in each category. This procedure tests the null hypothesis that the experimental data were derived from the same population as the control or theoretical population and that any differences in the proportion of data within individual categories are due to chance sampling.
A-squared (A2) refers to a numerical value produced by the Anderson-Darling test for normality. The test ultimately generates an approximate P-value where the null hypothesis is that the data are derived from a population that is normal. In the case of the data in , the conclusion is that there is
Some textbooks, particularly older ones, present a method known as the approach in conjunction with the -test. This method, which traditionally involves looking up -values in lengthy appendices, was developed long before computer software was available to calculate precise -values. Part of the reason this method may persist, at least in some textbooks, is that it provides authors with a vehicle to explain the basis of hypothesis testing along with certain technical aspects of the -test. As a modern method for analyzing data, however, it has long since gone the way of the dinosaur. Feel no obligation to learn this.
The rationale behind using the paired -test is that it takes meaningfully linked data into account when calculating the -value. The paired -test works by first calculating the difference between each individual pair. Then a mean and variance are calculated for all the differences among the pairs. Finally, a one-sample -test is carried out where the null hypothesis is that the mean of the differences is equal to zero. Furthermore, the paired -test can be one- or two-tailed, and arguments for either are similar to those for two independent means. Of course, standard programs will do all of this for you, so the inner workings are effectively invisible. Given the enhanced power of the paired -test to detect differences, it is often worth considering how the statistical analysis will be carried out at the stage when you are developing your experimental design. Then, if it's feasible, you can design the experiment to take advantage of the paired -test method.
By using Minitab as the statistical software and ANOVA as the inferential test, the first hypothesis was statistically proved by DES with a p-value of 0.038, such that these twenty couples who had completed the abridged Marriage CORE program were found to have a higher marital satisfaction.
The paired -test is a powerful way to detect differences in two sample means, provided that your experiment has been designed to take advantage of this approach. In our example of embryonic GFP expression, the two samples were in that the expression within any individual embryo was not linked to the expression in any other embryo. For situations involving independent samples, the paired -test is not applicable; we carried out an unpaired -test instead. For the paired method to be valid, data points must be linked in a meaningful way. If you remember from our first example, worms that have a mutation in show lower expression of the ::GFP reporter. In this example of a paired -test, consider a strain that carries a construct encoding a hairpin dsRNA corresponding to gene . Using a specific promoter and the appropriate genetic background, the dsRNA will be expressed only in the rightmost cell of one particular neuronal pair, where it is expected to inhibit the expression of gene via the RNAi response. In contrast, the neuron on the left should be unaffected. In addition, this strain carries the same ::GFP reporter described above, and it is known that this reporter is expressed in both the left and right neurons at identical levels in wild type. The experimental hypothesis is therefore that, analogous to what was observed in embryos, fluorescence of the ::GFP reporter will be weaker in the right neuron, where gene has been inhibited.
It is also worth pointing out that there is another way in which the -test could be used for this analysis. Namely, we could take the ratios from the first three blots (3.33, 3.41, and 2.48), which average to 3.07, and carry out a one-sample two-tailed -test. Because the null hypothesis is that there is no difference in the expression of protein X between wild-type and backgrounds, we would use an expected ratio of 1 for the test. Thus, the -value will tell us the probability of obtaining a ratio of 3.07 if the expected ratio is really one. Using the above data points, we do in fact obtain = 0.02, which would pass our significance cutoff. In fact, this is a perfectly reasonable use of the -test, even though the test is now being carried out on ratios rather than the unprocessed data. Note, however, that changing the numbers only slightly to 3.33, 4.51, and 2.48, we would get a mean of 3.44 but with a corresponding -value of 0.054. This again points out the problem with -tests when one has very small sample sizes and moderate variation within samples.