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CHI-SQUARE( X 2 ) DISTRIBUTION

CHI-SQUARE( X 2 ) DISTRIBUTION

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CHI-SQUARE( X 2 ) DISTRIBUTION

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  1. CHI-SQUARE(X2) DISTRIBUTION Chi-Square Test

  2. CHI-SQUARE(X2) DISTRIBUTION • PROPERTIES: 1.It is one of the most widely used distribution in statistical applications 2.This distribution may be derived from normal distribution 3.This distribution assumes values from ( zero to + infinity)

  3. CHI-SQUARE(X2) DISTRIBUTION 4. X2 relates to frequencies of occurrence of individuals (or events) in the categories of one or more variables. 5. X2 test used to test the agreement between the observed frequencies with certain characteristics and the expected frequencies under certain hypothesis.

  4. CHI-SQUARE(X2) DISTRIBUTION • CHI-SQUARE(X2) test of Goodness of fit • CHI-SQUARE(X2) test of homogeneity • CHI-SQUARE(X2) test of Independence

  5. CHI-SQUARE(X2) test of Independence • It is used to test the null hypothesis that two criteria of classification when applied to the same set of entities are independent (NO ASSOCIATION)

  6. CHI-SQUARE(X2) test of Independence • Generally , a single sample of size (n) can be drawn from a population, the frequency of occurrence of the entities are cross-classified on the basis of the two variables of interest( X &Y). The corresponding cells are formed by the intersections of the rows (r), and the columns (c). The table is called the ‘contingency table’

  7. CHI-SQUARE(X2) test of Independence • Calculation of expected frequency is based on the Probability Theory • The hypotheses and conclusions are stated on in terms of the independence or lack of independence of the two variables.

  8. CHI-SQUARE(X2) test of Independence • X2=∑(O-E)2/E • df=(r-1)(c-1) For 2x2 table, another formula to calculate X2 n(ad-bc)2 X2 =-------------------------------- (a+c)(b+d)(a+b)(c+d)

  9. Steps in constructing X2 -test • Hypotheses Ho: the 2 criteria are independent (no association) HA: The 2 criteria are not independent (There is association) 2. Construct the contingency table

  10. Steps in constructing X2 -test 3. Calculate the expected frequency for each cell By multiplying the corresponding marginal totals of that cell, and divide it by the sample size ∑E = ∑O for each row or column

  11. Steps in constructing X2 -test 4. Calculated the X2 value (calculated X2 c) X2=∑(O-E)2/E X2=∑(O-E)2/E For each cell we will calculate X2 value X2 value for all the cells of the contingency table will be added together to find X2 c

  12. Steps in constructing X2 -test 5. Define the critical value (tabulated X2) This depends on alpha level of significance and degree of freedom The value will be determined from X2 table df=(r-1)(c-1) r: no. of row c: no. of column

  13. Steps in constructing X2 -test 6. Conclusion If the X2 c is less than X2 tab we accept Ho. If the X2 c is more than X2 tab we reject Ho.

  14. Observed frequencies in a fourfold table

  15. For r X c table X2 –test is not applicable if: • The expected frequency of any cell is <1 • The expected frequencies of 20% of the cells is < 5

  16. For 2 X 2 table X2 –test is not applicable if: The expected frequency of any cell is <5

  17. EXERCISE • A group of 350 adults who participated in a health survey were asked whether or not they were on a diet. The response by sex are as follows

  18. EXERCISE

  19. EXERCISE • At alpha =0.05 do these data suggest an association between sex and being on diet?

  20. ANSWER • Ho: Being on diet and sex are independent ( no association) HA: Being on diet and sex are not independent ( there is association)

  21. 2. Calculation of expected frequencies 173 x 39 Cell a =-------------=19.3 350 177 x 39 Cell b=--------------=19.7 350

  22. 2. Calculation of expected frequencies 173 x 311 Cell c =-------------=153.7 350 177 x 311 Cell d=--------------=157.3 350

  23. Observed and (expected) frequencies

  24. ANSWER 3. Calculate X2 : X2=∑(O-E)2/E (14-19.3)2 (25-19.7)2 (159-153.7)2 (152-157.3)2 =-----------+-----------+--------------+------------- 19.3 19.7 153.7 157.3 =1.455+1.426+0.183+0.17 X2c =3.243

  25. ANSWER 4. Find X2 tab df= (r-1) (c-1)= (2-1)(2-1)=1 X20.95 df=1=3.841

  26. ANSWER 5. Conclusion Since X2 c < X2 tab we accept Ho ( No association between sex and being on diet)

  27. Another solution • Since this a 2x2 table we can use this formula: n(ad-bc)2 X2 =-------------------------------- (a+c)(b+d)(a+b)(c+d) 350{(14 x 152)-(25 x 159)}2 =------------------------------------- =3.22 39 x 311 x 173 x 177

  28. (Example) Five hundred elementary school children were cross classified by socioeconomic group and the presence or absence of a certain speech defect. The result were as follows

  29. Are these data compatible with the hypothesis that the speech defect is unrelated to socioeconomic status? • 1) Ho :Speech defect and SE group are independent ( no Association) • HA: Speech defect and SE group are not independent ( Association exist) • 2)Calculate the expected frequencies • 3)Calculate the X2 value ( calculated value)

  30. X² = ∑ (0 –E)² / E • X² = ∑ (8 – 9.1)² /9.1 + (24 – 26.4)²/26.4 + (32 – 30.9)² /30.9 + (27-24.6)² /34.6 + (121 – 118.6)²/118.6 + (138 -139.1)²/139.1 + (108 – 110.4)²/110.4 • X²=0.5 • Tab X² • DF = (2-1) (4-1) =3 → X²0.95 = 7.815

  31. (Example 2) • Five hundred employees of a factory that manufacture a product suspected of being associated with respiratory disorders were cross classified by level of exposure to the product and weather or not they exhibited symptoms of respiratory disorders. The results are shown in following table:

  32. Do these data provide sufficient evidence, at the 0.01 level of significance to indicate a relationship between level of exposure and the presence of respiratory and the presence of respiratory disorder ? • 1) Ho : The presence of respiratory symptoms and the level of exposure are independent. • HA : The two criteria are not independent • 2)Calculate the expected frequencies • 3) Calculate the X2

  33. X² =∑ (185 – 143.4)²/143.4 + (33 – 49.8)²/49.8 + (17-41.8)²/41.8 + (120-161.6)²/161.6 + (73 -56.2)² /56.2 + (72-47.2)²/47.2 = 33.47 Tab X² 0.99 = 9.21 Reject Ho • Df = (3-1) (2-1) = 2

  34. (Example 3) • In a clinical trial involving a potential hypothesis drug, patients are assigned at random either to receive the active drug or placebo. The trial is double blind, that is neither the patient nor the examining physician knows with of the 2 treatment the patient is receiving. Patients response to treatment is categorized as favorable or unfavorable on the basis of degree and duration of response in BP. There are 50 patients assigned to each group.

  35. X² = n (ad – bc)²/(a+b)(c+d)(a+c)(b+d) • = 100[(34x41) – (9x16)]²/(50) (50)(43)(57) • =25.5

  36. (Example 4) • A study found that mongolism in babies is associated with hepatitis A injection of the mother during pregnancy. Suppose a study of 2000 randomly selected mothers to be yielded the following table after the births of their babies.

  37. what is the conclusion.