7  Reliabilitas dan Validitas

7.1 Reliabilitas

pretest <- c(68, 74, 71, 85, 68, 58, 90, 75,52, 72)
posttest <- c(82, 85, 70, 88, 83, 60, 85, 75,50, 70)
cor(pretest, posttest)
[1] 0.8183822

7.2 Alfa Cronbach’s

\[ \alpha = (\frac{k}{k-1})(\frac{s^2_y-\Sigma s_i^2}{s_y^2}) \]

dengan

  • \(k\) = jumlah item

  • \(s_y^2\) = variansi skor yang diamati

  • \(\Sigma s_i^2\) = jumlah semua variansi item

ID

Item_1

Item_2

Item_3

Item_4

Item_5

Total

1

3.0

2

2

3.0

2.0

12

2

3.0

2

2

3.0

3.0

13

3

4.0

2

2

4.0

2.0

12

4

3.0

2

2

2.0

3.0

12

5

2.0

2

2

3.0

2.0

11

6

3.0

2

2

3.0

2.0

12

7

4.0

2

2

3.0

1.0

12

8

3.0

2

2

3.0

3.0

13

9

3.0

2

2

1.0

3.0

11

10

3.0

2

2

3.0

2.0

12

Variansi

0.3

0

0

0.6

0.5

\[ \alpha = (\frac{5}{5-1})(\frac{120-0.41}{120})=1.25 \]

7.3 Menghitung Alfa Cronbach’s Menggunakan R

Item_1

Item_2

Item_3

Item_4

Item_5

3

2

2

3

2

3

2

2

3

3

4

2

2

4

2

3

2

2

2

3

2

2

2

3

2

3

2

2

3

2

4

2

2

3

1

3

2

2

3

3

3

2

2

1

3

3

2

2

3

2

library(psych)
alpha(pisa)
Warning in alpha(pisa): Some items were negatively correlated with the first principal component and probably 
should be reversed.  
To do this, run the function again with the 'check.keys=TRUE' option
Some items ( Item_2 Item_3 Item_5 ) were negatively correlated with the first principal component and 
probably should be reversed.  
To do this, run the function again with the 'check.keys=TRUE' option

Reliability analysis   
Call: alpha(x = pisa)

  raw_alpha std.alpha G6(smc) average_r     S/N    ase mean   sd median_r
   -0.0037   -0.0069    0.21   -0.0014 -0.0068 0.0031  2.5 0.29   -0.029

    95% confidence boundaries 
         lower alpha upper
Feldt    -0.01     0     0
Duhachek -0.01     0     0

 Reliability if an item is dropped:
       raw_alpha std.alpha G6(smc) average_r     S/N alpha se var.r   med.r
Item_1    0.0841    0.0574    0.17    0.0150  0.0609   0.0027 0.059 -0.0016
Item_2    0.0298    0.0577    0.19    0.0151  0.0612   0.0030 0.067 -0.0289
Item_3   -0.0021   -0.0054    0.26   -0.0014 -0.0054   0.0032 0.144 -0.1849
Item_4   -0.0581   -0.0754    0.10   -0.0179 -0.0702   0.0032 0.075 -0.0188
Item_5   -0.0852   -0.0748    0.13   -0.0177 -0.0696   0.0033 0.089 -0.0289

 Item statistics 
            n raw.r std.r   r.cor  r.drop mean   sd
Item_1 287354  0.38  0.40  0.2031 -0.0687  3.2 0.65
Item_2 287354  0.42  0.40  0.1665 -0.0262  1.9 0.65
Item_3 287354  0.41  0.45 -0.0019 -0.0021  2.0 0.60
Item_4 287354  0.47  0.49  0.3549  0.0409  3.2 0.62
Item_5 287354  0.54  0.49  0.2987  0.0514  2.2 0.71

Non missing response frequency for each item
          1    2    3    4 miss
Item_1 0.02 0.08 0.61 0.29    0
Item_2 0.25 0.63 0.10 0.02    0
Item_3 0.17 0.70 0.11 0.02    0
Item_4 0.02 0.08 0.64 0.27    0
Item_5 0.12 0.61 0.21 0.05    0

7.4 Interrater Reliability

\[ Interrater reliability =\frac{Number of agreements}{Number of possible agreements} \]