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Showing posts with label hypothesis test. Show all posts
Showing posts with label hypothesis test. Show all posts

Sunday, August 25, 2019

Two Sample t-test

Two sample t-test

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x = c(70, 82, 78, 74, 94, 82) 
> n = length(x)
> m=8 observation of y
"m=8 observation"
> y = c(64, 72, 60, 76, 72, 80, 84, 68)
> m = length(y)
> we will test H0 : µ1 = µ2 versus H1 : µ1 > µ2.
> x_bar = mean(x)
> s_x = sd(x)
> y_bar = mean(y)
> s_y = sd(y)
> s_p = sqrt(((n - 1) * s_x ^ 2 + (m - 1) * s_y ^ 2) / (n + m - 2))
> t = ((x_bar - y_bar) - 0) / (s_p * sqrt(1 / n + 1 / m))
> t
[1] 1.823369
> 1 - pt(t, df = n + m - 2)
[1] 0.04661961
> t.test(x, y, alternative = c("greater"), var.equal = TRUE)

Two Sample t-test


data:  x and y
t = 1.8234, df = 12, p-value = 0.04662
alternative hypothesis: true difference in means is greater than 0
95 percent confidence interval:
 0.1802451       Inf
sample estimates:
mean of x mean of y 
       80        72  
> t_test_data = data.frame(values = c(x, y), group = c(rep("A", length(x)), rep("B", length(y))))
> t_test_data
   values group
1      70     A
2      82     A
3      78     A
4      74     A
5      94     A
6      82     A
7      64     B
8      72     B
9      60     B
10     76     B
11     72     B
12     80     B
13     84     B
14     68     B
> t.test(values ~ group, data = t_test_data, alternative = c("greater"), var.equal = TRUE)

https://youtu.be/DwKq404Q0Gk

Friday, August 23, 2019

Structure of the hypothesis test

What is the basic structure of the hypothesis test?

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The most common being observation following a normal distribution.
The ᕼo(Null) and Ⱨ(alternative hypothesis are specified) mostly null specifies a particular value of a parameter.
Under the general assumption, we take Ho is true, the distribution of the test statistic is known.

Given the distribution and value of the test statistic and the form of Ⱨ, we can calculate the P-value of the test.
Based on p-value and pre-specified level of significance, we make decision
Fail to reject Ho 
Reject the Ho

One sample t-test in R

 we have taken 9 random sample 
 t.test(x = apt_crisp$weight, mu = 16, alternative = c("less"), conf.level = 0.95)
> apt_crisp = data.frame(weight = c(15.5, 16.2, 16.1, 15.8, 15.6, 16.0, 15.8, 15.9, 16.2))
> x_bar = mean(apt_crisp$weight)
> s = sd(apt_crisp$weight)
> mu_0 = 16 
> n = 9
> t = (x_bar - mu_0) / (s / sqrt(n))
> t
[1] -1.2
> pt(t, df = n - 1)
[1] 0.1322336
data:  apt_crisp$weight
t = -1.2, df = 8, p-value = 0.1322
alternative hypothesis: true mean is less than 16
95 percent confidence interval:
     -Inf 16.05496
sample estimates:
mean of x 
     15.9 

> apt_test_results = t.test(apt_crisp$weight, mu = 16,
+ alternative = c("two.sided"), conf.level = 0.95)
> names(apt_test_results)
 [1] "statistic"   "parameter"   "p.value"    
 [4] "conf.int"    "estimate"    "null.value" 
 [7] "stderr"      "alternative" "method"     
[10] "data.name"  
> qt(0.975, df = 8)
[1] 2.306004
apt_test_results$conf.int
[1] 15.70783 16.09217
attr(,"conf.level")
[1] 0.95
> c(mean(apt_crisp$weight) - qt(0.975, df = 8) * sd(apt_crisp$weight) / sqrt(9),
+   mean(apt_crisp$weight) + qt(0.975, df = 8) * sd(apt_crisp$weight) / sqrt(9))
[1] 15.70783 16.09217

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