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# How To Remove Unnecessary Variables For A Model In R

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## Introduction

In this article I am going to demonstrate how to remove irrelevant variables from datasets for creating a model in R. Using a combination of function aggregate with certain variables and datasets, we can select and remove irrelevant variables from a dataset to create a model in R. Using combination function, we can exclude several irrelevant variables to create a model.

## Removing the variables

In order to remove multiple variables together to fit a model, we can use a combination function along with mathematical operators so as to remove two or more than two variables together. For example, we can use subtraction operator inside aggregate function to remove multiple variables for the creation of a model. To create a model in R, we can use various mathematical operators to remove irrelevant variables.

We can define a formula along with arithmetic operators in lots of functions in R. One such functions is the aggregate() function using which we can remove different irrelevant variables to create a model.

Now I will demonstrate the use of cluster function to remove several variables together. We will be using gscars dataset to demonstrate the use of aggregate function.
1. > gscars
2.                      mg cyl  disp  hp drat    wt  qsec vs bn gr carb
3. Mazda RX4           21.0   6 160.0 110 3.90 2.620 16.46  0  1    4    4
4. Mazda RX4 Wag       21.0   6 160.0 110 3.90 2.875 17.02  0  1    4    4
5. Datsun 710          22.8   4 108.0  93 3.85 2.320 18.61  1  1    4    1
6. Hornet 4 Drive      21.4   6 258.0 110 3.08 3.215 19.44  1  0    3    1
7. Hornet Sportabout   18.7   8 360.0 175 3.15 3.440 17.02  0  0    3    2
8. Valiant             18.1   6 225.0 105 2.76 3.460 20.22  1  0    3    1
9. Duster 360          14.3   8 360.0 245 3.21 3.570 15.84  0  0    3    4
10. Merc 240D           24.4   4 146.7  62 3.69 3.190 20.00  1  0    4    2
11. Merc 230            22.8   4 140.8  95 3.92 3.150 22.90  1  0    4    2
12. Merc 280            19.2   6 167.6 123 3.92 3.440 18.30  1  0    4    4
13. Merc 280C           17.8   6 167.6 123 3.92 3.440 18.90  1  0    4    4
14. Merc 450SE          16.4   8 275.8 180 3.07 4.070 17.40  0  0    3    3
15. Merc 450SL          17.3   8 275.8 180 3.07 3.730 17.60  0  0    3    3
16. Merc 450SLC         15.2   8 275.8 180 3.07 3.780 18.00  0  0    3    3
17. Cadillac Fleetwood  10.4   8 472.0 205 2.93 5.250 17.98  0  0    3    4
18. Lincoln Continental 10.4   8 460.0 215 3.00 5.424 17.82  0  0    3    4
19. Chrysler Imperial   14.7   8 440.0 230 3.23 5.345 17.42  0  0    3    4
20. Fiat 128            32.4   4  78.7  66 4.08 2.200 19.47  1  1    4    1
21. Honda Civic         30.4   4  75.7  52 4.93 1.615 18.52  1  1    4    2
22. Toyota Corolla      33.9   4  71.1  65 4.22 1.835 19.90  1  1    4    1
23. Toyota Corona       21.5   4 120.1  97 3.70 2.465 20.01  1  0    3    1
24. Dodge Challenger    15.5   8 318.0 150 2.76 3.520 16.87  0  0    3    2
25. AMC Javelin         15.2   8 304.0 150 3.15 3.435 17.30  0  0    3    2
26. Camaro Z28          13.3   8 350.0 245 3.73 3.840 15.41  0  0    3    4
27. Pontiac Firebird    19.2   8 400.0 175 3.08 3.845 17.05  0  0    3    2
28. Fiat X1-9           27.3   4  79.0  66 4.08 1.935 18.90  1  1    4    1
29. Porsche 914-2       26.0   4 120.3  91 4.43 2.140 16.70  0  1    5    2
30. Lotus Europa        30.4   4  95.1 113 3.77 1.513 16.90  1  1    5    2
31. Ford Pantera L      15.8   8 351.0 264 4.22 3.170 14.50  0  1    5    4
32. Ferrari Dino        19.7   6 145.0 175 3.62 2.770 15.50  0  1    5    6
33. Maserati Bora       15.0   8 301.0 335 3.54 3.570 14.60  0  1    5    8
34. Volvo 142E          21.4   4 121.0 109 4.11 2.780 18.60  1  1    4    2
35. >
We will be using ~ operator inside aggregate function along with subtraction operator to remove multiple irrelevant variables in R. For example if there are three variables named a, b and c, then a ~ b - c means that aggregate function will create a model a, which will contain variable b excluding variable c.
1. > aggregate(mg ~ gr - bn, data = data, mean)
2.   gr bn      mg
3. 1    4  0 15.10667
4. 2    5  0 23.05000
5. 3    5  1 22.27500
6. 4    6  1 24.38000
In the above aggregate function, there are three arguments. The first argument in the formula indicates that aggregate function represents mg as a function by including gr and removing bn variable calculating the mean and the second argument is variable depicting the dataset. Using above code, aggregate function creates a model in which mg variable will act as a function of gr variable and bn variable will be removed from the model.
1. > aggregate(mg ~ disp - hp, data = data, mean)
2.     disp  hp  mg
3. 1   75.7  52 30.4
4. 2  146.7  62 24.4
5. 3   71.1  65 33.9
6. 4   78.7  66 32.4
7. 5   79.0  66 27.3
8. 6  120.3  91 26.0
9. 7  108.0  93 22.8
10. 8  140.8  95 22.8
11. 9  120.1  97 21.5
12. 10 225.0 105 18.1
13. 11 121.0 109 21.4
14. 12 160.0 110 21.0
15. 13 258.0 110 21.4
16. 14  95.1 113 30.4
17. 15 167.6 123 18.5
18. 16 304.0 150 15.2
19. 17 318.0 150 15.5
20. 18 145.0 175 19.7
21. 19 360.0 175 18.7
22. 20 400.0 175 19.2
23. 21 275.8 180 16.3
24. 22 472.0 205 10.4
25. 23 460.0 215 10.4
26. 24 440.0 230 14.7
27. 25 350.0 245 13.3
28. 26 360.0 245 14.3
29. 27 351.0 264 15.8
30. 28 301.0 335 15.0
In the above aggregate function, there are three arguments. First argument in the formula indicates that aggregate function represents mg as a function by including disp and removing hp variable calculating the mean and the second argument is variable depicting the dataset. Using the above code, aggregate function creates a model in which mg variable will act as a function of disp variable and hp variable will be removed from the model.
1. > aggregate(hp ~ mg - cyl, data = data, mean)
2.     mg cyl    hp
3. 1  21.4   4 109.0
4. 2  21.5   4  97.0
5. 3  22.8   4  94.0
6. 4  24.4   4  62.0
7. 5  26.0   4  91.0
8. 6  27.3   4  66.0
9. 7  30.4   4  82.5
10. 8  32.4   4  66.0
11. 9  33.9   4  65.0
12. 10 17.8   6 123.0
13. 11 18.1   6 105.0
14. 12 19.2   6 123.0
15. 13 19.7   6 175.0
16. 14 21.0   6 110.0
17. 15 21.4   6 110.0
18. 16 10.4   8 210.0
19. 17 13.3   8 245.0
20. 18 14.3   8 245.0
21. 19 14.7   8 230.0
22. 20 15.0   8 335.0
23. 21 15.2   8 165.0
24. 22 15.5   8 150.0
25. 23 15.8   8 264.0
26. 24 16.4   8 180.0
27. 25 17.3   8 180.0
28. 26 18.7   8 175.0
29. 27 19.2   8 175.0
In the above aggregate function, there are three arguments. First argument in the formula indicates that aggregate function represents hp as a function by including mg and removing cyl variable calculating the mean and the second argument is variable depicting the dataset. Using the above code, aggregate function creates a model in which hp variable will act as a function of mg variable and cyl variable will be removed from the model.
1. > aggregate(wt ~ gr - qsec, data = data, mean)
2.    gr  qsec     wt
3. 1     5 14.50 3.1700
4. 2     5 14.60 3.5700
5. 3     3 15.41 3.8400
6. 4     5 15.50 2.7700
7. 5     3 15.84 3.5700
8. 6     4 16.46 2.6200
9. 7     5 16.70 2.1400
10. 8     3 16.87 3.5200
11. 9     5 16.90 1.5130
12. 10    3 17.02 3.4400
13. 11    4 17.02 2.8750
14. 12    3 17.05 3.8450
15. 13    3 17.30 3.4350
16. 14    3 17.40 4.0700
17. 15    3 17.42 5.3450
18. 16    3 17.60 3.7300
19. 17    3 17.82 5.4240
20. 18    3 17.98 5.2500
21. 19    3 18.00 3.7800
22. 20    4 18.30 3.4400
23. 21    4 18.52 1.6150
24. 22    4 18.60 2.7800
25. 23    4 18.61 2.3200
26. 24    4 18.90 2.6875
27. 25    3 19.44 3.2150
28. 26    4 19.47 2.2000
29. 27    4 19.90 1.8350
30. 28    4 20.00 3.1900
31. 29    3 20.01 2.4650
32. 30    3 20.22 3.4600
33. 31    4 22.90 3.1500
In the above aggregate function, there are three arguments. First argument in the formula indicates that aggregate function represents wt as a function by including gr and removing qsec variable calculating the mean and the second argument is variable depicting the dataset. Using above code, aggregate function creates a model in which wt variable will act as a function of gr variable and qsec variable will be removed from the model.

## Summary

In this article I demonstrated how to remove irrelevant variables from dataset for creating a model in R. Using a combination of function aggregate with certain variables and datasets, we can select and remove irrelevant variables from a dataset to create a model in R.

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