WEBVTT 1 00:00:00.125 --> 00:00:01.980 Hello students. 2 00:00:01.980 --> 00:00:06.870 Welcome to Biostatistics ER, Example 4, Chapter 7. 3 00:00:06.870 --> 00:00:08.970 In this example, we are going to discuss 4 00:00:08.970 --> 00:00:10.950 how to determine p value 5 00:00:10.950 --> 00:00:13.800 when we reject the null hypothesis. 6 00:00:13.800 --> 00:00:15.330 Now, please do keep in mind 7 00:00:15.330 --> 00:00:16.470 that when we do use 8 00:00:16.470 --> 00:00:19.710 a statistical software package like SPSS, 9 00:00:19.710 --> 00:00:22.080 the p value is generated for us, 10 00:00:22.080 --> 00:00:23.673 which is an exact p value. 11 00:00:24.780 --> 00:00:27.720 This is not something we can determine easily 12 00:00:27.720 --> 00:00:30.900 when we are calculating the p value by hand 13 00:00:30.900 --> 00:00:32.223 using the t table. 14 00:00:33.750 --> 00:00:36.240 But we can have an approximate idea 15 00:00:36.240 --> 00:00:37.743 where the p value will be. 16 00:00:39.930 --> 00:00:42.210 So now we will learn how to do that 17 00:00:42.210 --> 00:00:44.580 and we will go back to example 3, 18 00:00:44.580 --> 00:00:49.580 in which we learned that our decision rule 19 00:00:52.710 --> 00:00:56.070 was that we would reject the null hypothesis 20 00:00:56.070 --> 00:00:58.620 if our t calculated was less than 21 00:00:58.620 --> 00:01:01.740 or equal to negative 1.995 22 00:01:01.740 --> 00:01:04.110 or if our t calculated was greater than 23 00:01:04.110 --> 00:01:07.473 or equal to positive 1.995. 24 00:01:08.670 --> 00:01:12.660 Now our t calculated here was negative 7.77, 25 00:01:12.660 --> 00:01:14.850 so we could reject the null hypothesis 26 00:01:14.850 --> 00:01:17.970 because negative 7.77 27 00:01:17.970 --> 00:01:22.170 is less than negative 1.995. 28 00:01:22.170 --> 00:01:24.423 So now the question is what is the p value? 29 00:01:25.290 --> 00:01:27.660 So now we will go to the back of our textbook 30 00:01:27.660 --> 00:01:31.380 and look at the degrees of freedom equal to 68, 31 00:01:31.380 --> 00:01:33.030 and if we go down that row, 32 00:01:33.030 --> 00:01:35.943 that has the degrees of freedom equal to 68, 33 00:01:37.560 --> 00:01:39.780 we look at a two-tailed significance 34 00:01:39.780 --> 00:01:43.710 with a level of 0.05, 35 00:01:43.710 --> 00:01:48.130 we can see that 1.995 36 00:01:50.730 --> 00:01:54.450 is the t critical value we used. 37 00:01:54.450 --> 00:01:57.030 Now we need to find a number that is close 38 00:01:57.030 --> 00:01:58.950 to our t calculated 39 00:01:58.950 --> 00:02:03.070 and we see that that value does not exist in our table 40 00:02:05.370 --> 00:02:08.073 for degrees of freedom equal to 68. 41 00:02:09.000 --> 00:02:11.850 So we can state that the p value 42 00:02:11.850 --> 00:02:14.820 will be less than 0.01, 43 00:02:14.820 --> 00:02:17.940 which is the lowest p value we have 44 00:02:17.940 --> 00:02:22.923 for a two-sided test with degrees of freedom equal to 68. 45 00:02:25.020 --> 00:02:27.300 Now just to clarify, 46 00:02:27.300 --> 00:02:30.330 we will go over another example. 47 00:02:30.330 --> 00:02:33.090 And in this example we will assume 48 00:02:33.090 --> 00:02:36.270 that the degrees of freedom is equal to 32 49 00:02:36.270 --> 00:02:39.813 and our t calculated is equal to 2.12. 50 00:02:40.770 --> 00:02:43.020 Now for the t critical value, 51 00:02:43.020 --> 00:02:47.743 we can see for a two-sided test at alpha equal to 0.05, 52 00:02:49.560 --> 00:02:52.833 the value is 2.037, 53 00:02:53.700 --> 00:02:56.430 and the next t critical value 54 00:02:56.430 --> 00:02:59.133 after that is 2.449. 55 00:03:00.060 --> 00:03:02.640 Now our t calculated is in between 56 00:03:02.640 --> 00:03:04.950 these two critical t values. 57 00:03:04.950 --> 00:03:07.050 So here we can only state 58 00:03:07.050 --> 00:03:10.113 that our p value is less than 0.05. 59 00:03:12.750 --> 00:03:15.930 I understand that this example is a little bit different 60 00:03:15.930 --> 00:03:17.760 from the previous examples 61 00:03:17.760 --> 00:03:20.163 that we have covered for Chapter 7. 62 00:03:21.780 --> 00:03:24.300 So if this comes across a little bit challenging, 63 00:03:24.300 --> 00:03:27.153 please let me know because I'm here to help. 64 00:03:28.200 --> 00:03:30.720 And again, thank you for your time and attention 65 00:03:30.720 --> 00:03:32.823 and I will see you in the next video.