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Using Contrasts in One-Way Analysis of Variance with Control Groups and an Application

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Using Contrasts in One-Way Analysis of Variance with Control Groups and an Application Journal of Agricultural Science and Technology A 7 (2017) 474-478 doi: 10.17265/2161-6256/2017.07.003 D DAVID PUBLISHING Using Contrasts in One-Way Analysis of Variance with Control Groups and an Application Demet Çanga1 and Ceren Efe2 1. Department of Food Processing, University of Osmaniye Korkut Ata, Osmaniye 80050, Turkey 2. Department of Biostatistics, Faculty of Medicine, Çukurova University, Adana 01330, Turkey Abstract: In this study, it was shown that, same comparisons can be made by using contrast coefficients instead of Dunnett’s test in the experiments with control groups. It was also shown that, in situations with an ordinal scale and equal spacing quantitative grouping, a trend investigation could be done by contrast coefficients. For this purpose, a small part of the data from a TUBITAK project in the Soil Science Department, Agriculture Faculty, Kahramanmaras Sutcu Imam University, was used with permission. The soils were absorbed to natural zeolite in concentration of 0, 2.5, 5 and 10 mg/kg, and after two years, the available Zinc (Zn) amounts in the soil were analyzed. As a result, in both Dunnett’s test and contrast methods, the Zn amounts in control and 2.5 mg/kg concentration groups were found similar (P > 0.01); but were different (P < 0.01) between control and 5 mg/kg concentration groups, and control and 10 mg/kg concentration groups. Furthermore, when orthogonal polynomial contrast coefficients were investigated, linear effects were found significant (P < 0.01) and cubic effects were obtained significant (P < 0.05), but quadratic effect was obtained insignificant (P > 0.05). Key words: Analysis of variance (ANOVA), comparisons, Dunnett’s test, contrast, trend analysis. 1. Introduction tests, such as Duncan’s and Tukey’s. In experiments with a control group, if the researcher desires only to In experimental research with more than two groups, compare the control group with the other groups, one-way analysis of variance (ANOVA) is a common Dunnett’s test is preferred [2, 5, 6]. These three tests method to compare means [1]. In analysis of variance, are called post-hoc (or unplanned) tests. the null hypothesis indicating that the group means are On the other hand, if the researchers would like to equal is tested using F test. If the null hypothesis is investigate the pairwise relations between particular rejected, further investigations to find out the pair groups instead of all the groups, they should use causing the difference are conducted by using multiple planned comparisons, for which the contrast comparison tests [2]. These tests are divided into two coefficients of the hypotheses are needed to be defined main categories called unplanned (post-hoc or [7, 8]. posteriori) and planned (priori) tests [3]. Dunnett’s In this study, the usage of contrast in one-way test, which is one of the unplanned comparisons, is ANOVA with control groups is thoroughly examined. used to compare the control group with all other Usage of contrast enables the researchers to consider groups [4]. the possible relationships between the groups more If one finds a difference between treatment means detailed [9, 10]. It is an alternative way of finding the by using F test, a detailed investigation can be carried difference between specific pairs of groups faster and out to determine the particular pair of groups that therefore a faster decision-making [11-13]. There are causes the difference, by using multiple comparison not many/enough publications about the usage of contrast methods. The aim of this study is to present Corresponding author: Demet Çanga, Ph.D., research that it is possible to make the same comparisons by fields: biometry and genetics, statistic. Using Contrasts in One-Way Analysis of Variance with Control Groups 475 and an Application using planned contrasts in place of Dunnett’s test, relationship between response and treatment for the which is a post-hoc test used for studies with control one-way ANOVA (Table 2) is given by the following groups. Eq. (1): (1) 2. Materials and Methods where, Yij: the j-th observation (j = 1, 2, ..., ni) on the 2.1 Materials i-th treatment (i = 1, 2, ..., k levels); : the common effect for the whole experiment; : the i-th treatment Dataset is obtained from a project carried out in the effect; eij: the random error present in the j-th Department of Soil Science, Faculty of Agriculture, observation on the i-th treatment [2, 6, 14]. Kahramanmaras Sutcu Imam University. It consists of Instead of comparing all the groups pairwise, each available zinc (Zn) amounts of the soils. In the project, group’s mean is compared only to the control group’s Zn sorption of natural zeolite and intake of wheat mean using Dunnett’s test. Dunnett’s test makes this plants are investigated. Project is conducted as a comparison using a single critical value, like least completely randomized one-way experimental design significant difference (LSD), Tukey’s test [5]. There with three replications. If a significant difference is are tD tables for one-way and two-way comparisons. If detected as a result of ANOVA, instead of a multiple the number of repetition of control and other groups pairwise comparison of all the groups in the are equal, Dunnett’s test statistic is as Eq. (2): experiment, Dunnett’s test, that is a test for comparing 2S 2 each group individually with the control group, is used. d=tD (2) And Dunnett’s test results are evaluated along with n where, tD: t-Dunnett table value with v degrees of the contrast method results. freedom of error; S2: mean square of error; n: number For this purpose, the soil obtained from of repetition. Kahramanmaras province was applied of natural Two-sided tD value is set as tD = tp, v, 1 - α = t3, 8, 1 - 0.05 = zeolite containing 0 (D-0), 2.5 (D-2.5), 5 (D-5) and 10 t3, 8, 0.995 = 2.88 for the test statistic used in the mg/kg (D-10) Zn, respectively. Obtained data of the comparisons for the difference of control group and the four groups are provided in Table 1. other means. (p: number of groups, except control; : significance level and v: degrees of freedom of error). 2.2 Methods This value is substituted in Eq. (2) and the obtained The mathematical model that describes the result is compared to the means of other groups [2, 5]. Table 1 Experiment/Trial results of natural zeolite affiliated with Zn (mg/kg). Replications D-0 (control) D-2.5 D-5 D-10 1 0.97 1.48 2.06 2.79 2 1.13 1.14 2.31 2.69 3 1.2 1.24 2.28 2.36 Mean 1.10 1.29 2.22 2.61 Table 2 ANOVA results of natural zeolite affiliated with Zn. Source of variation (SV) Degrees of freedom (DF) Sum of squares (SS) Mean squares (MS) F Between groups 3 4.7657 1.5886 55.8862** Within groups (error) 8 0.2274 0.0284 Total 11 4.9931 ** Significant at the 0.01 probability level F0.05 (3, 8) = 4.07, F0.05 (1, 8) = 5.32, F0.01 (1, 8) = 11.26. 476 Using Contrasts in One-Way Analysis of Variance with Control Groups and an Application If the same comparisons are desired to be made coefficients; n: number of conditions (or groups) using contrast coefficients, the researcher asks the repetition [15-18]. three questions below: 3. Results and Discussion (1) Is there any difference between control group and D-2.5 mg/kg group means? ANOVA results of the data in Table 1 are given in (2) Is there any difference between control group Table 2. From the ANOVA results in Table 2, it is and D-5 mg/kg group means? concluded that there is a statistically significant (3) Is there any difference between control group difference between the group means (P < 0.05). And and D-10 mg/kg group means? Dunnett’s test results showed that the means of group These hypotheses can be expressed also as following: D-5 and D-10 are different from the control group’s (1) H0, 1: µcontrol – µD-2.5 = 0 mean (P < 0.05), whereas the mean of the group D-2.5 (2) H0, 2: µcontrol – µD-5 = 0 was found to be equal to the control group’s mean (P > (3) H0, 3: µcontrol – µD-10 = 0 0.05). Contrast coefficients for these hypotheses and Results of contrast analysis are summarized in group means are summarized in Table 3. According to Table 4. When Table 4 is examined, it is seen that Table 3, sum of squares (SS) for the i-th contrast similar results are obtained with the Dunnett’s definition is calculated as Eq. (3): test. The difference between the means of the groups D-5 and control, D-10 and control is found significant nMC ()2 SS SS ii (3) (P < 0.01); however it is not significant between contrast estimation Ci C 2 i D-2.5 and control group (P > 0.05). This means where, SSCi: the sum of square of contrasts; Mi: that the Zn application must be at least 5 means of conditions (or groups); : contrast mg/kg. Table 3 Group means and contrast coefficients of the hypotheses. Coefficients Control D-2.5 D-5 D-10 Mean 1.10 1.29 2.22 2.61 2 ∑Ci ∑Ci C1 1 -1 0 0 0 2 C2 1 0 -1 0 0 2 C3 1 0 0 -1 0 2 2 ∑Mi Ci (∑Mi Ci) Mi C1 1.10 -1.29 0.00 0.00 -0.18 0.0324 Mi C2 1.10 0.00 -2.22 0.00 -1.12 1.2544 Mi C3 1.10 0.00 0.00 -2.61 -1.51 2.2801 Table 4 Contrast analysis results.
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