I’m giving a talk for the Kansas City R Users Group on how to get a preliminary impression of relationships between pairs of variables. Here is the R code and output that I will use.

Simple measures of association

There are several different ways of measuring bivariate relationships in a descriptive fashion prior to data analysis. The methods can be largely grouped into measures of relationship between two continuous variables, two categorical variables and measures of a relationship between a categorical variable and a continuous variable.

fn <- "http://lib.stat.cmu.edu/DASL/Datafiles/homedat.html"
h0 <- readLines(fn)
n <- length(h0)
print(n)
## [1] 165
head(h0)
## [1] "<TITLE>Home Prices</TITLE>"                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    
## [2] ""                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              
## [3] ""                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              
## [4] "<hr size=2><center><table border=1 cellpadding=0 cellspacing=0><tr><td align=center><table border=1 cellpadding=1 cellspacing=0><tr><td><A HREF="../DataArchive.html"><IMG SRC="../InlineImages/mainmenu.gif" alt="Go to Main Menu"></a></td></tr></table></td><td align=center><table border=1 cellpadding=1 cellspacing=0><tr><td><A HREF="/cgi-bin/dasl.cgi"><IMG SRC="../InlineImages/powersearchsmall.gif" alt="Go to Power Search"></a></td></tr></table></td><td align=center><table border=1 cellpadding=1 cellspacing=0><tr><td><A HREF="../allsubjects.html"><IMG SRC="../InlineImages/allsubjects.gif" alt="Go to Datafile Subjects"></a></td></tr></table></td></tr></table></center><hr size=2>"
## [5] "<B><DT>Datafile Name:</B>   Home Prices"                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       
## [6] ""
tail(h0)
## [1] "872\t1229\t6\t3\t0\t0\t0\t721" "870\t1273\t4\t4\t0\t0\t0\t638"
## [3] "869\t1165\t7\t4\t0\t0\t0\t694" "766\t1200\t7\t4\t0\t0\t1\t634"
## [5] "739\t970\t4\t4\t0\t0\t1\t541"  "</PRE>"
the.data.line <- grep("The Data:",h0)
print(the.data.line)
## [1] 44
h1 <- h0[(the.data.line+4):(n-1)]
head(h1)
## [1] "2050\t2650\t13\t7\t1\t1\t0\t1639" "2080\t2600\t*\t4\t1\t1\t0\t1088" 
## [3] "2150\t2664\t6\t5\t1\t1\t0\t1193"  "2150\t2921\t3\t6\t1\t1\t0\t1635" 
## [5] "1999\t2580\t4\t4\t1\t1\t0\t1732"  "1900\t2580\t4\t4\t1\t0\t0\t1534"
tail(h1)
## [1] "874\t1236\t3\t4\t0\t0\t0\t707" "872\t1229\t6\t3\t0\t0\t0\t721"
## [3] "870\t1273\t4\t4\t0\t0\t0\t638" "869\t1165\t7\t4\t0\t0\t0\t694"
## [5] "766\t1200\t7\t4\t0\t0\t1\t634" "739\t970\t4\t4\t0\t0\t1\t541"
h2 <- strsplit(h1,"\t")
head(h2)
## [[1]]
## [1] "2050" "2650" "13"   "7"    "1"    "1"    "0"    "1639"
## 
## [[2]]
## [1] "2080" "2600" "*"    "4"    "1"    "1"    "0"    "1088"
## 
## [[3]]
## [1] "2150" "2664" "6"    "5"    "1"    "1"    "0"    "1193"
## 
## [[4]]
## [1] "2150" "2921" "3"    "6"    "1"    "1"    "0"    "1635"
## 
## [[5]]
## [1] "1999" "2580" "4"    "4"    "1"    "1"    "0"    "1732"
## 
## [[6]]
## [1] "1900" "2580" "4"    "4"    "1"    "0"    "0"    "1534"
tail(h2)
## [[1]]
## [1] "874"  "1236" "3"    "4"    "0"    "0"    "0"    "707" 
## 
## [[2]]
## [1] "872"  "1229" "6"    "3"    "0"    "0"    "0"    "721" 
## 
## [[3]]
## [1] "870"  "1273" "4"    "4"    "0"    "0"    "0"    "638" 
## 
## [[4]]
## [1] "869"  "1165" "7"    "4"    "0"    "0"    "0"    "694" 
## 
## [[5]]
## [1] "766"  "1200" "7"    "4"    "0"    "0"    "1"    "634" 
## 
## [[6]]
## [1] "739" "970" "4"   "4"   "0"   "0"   "1"   "541"
h3 <- as.numeric(unlist(h2))
## Warning: NAs introduced by coercion
head(h3)
## [1] 2050 2650   13    7    1    1
tail(h3)
## [1]   4   4   0   0   1 541
h4 <- matrix(h3,ncol=8,byrow=TRUE)
head(h4)
##      [,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8]
## [1,] 2050 2650   13    7    1    1    0 1639
## [2,] 2080 2600   NA    4    1    1    0 1088
## [3,] 2150 2664    6    5    1    1    0 1193
## [4,] 2150 2921    3    6    1    1    0 1635
## [5,] 1999 2580    4    4    1    1    0 1732
## [6,] 1900 2580    4    4    1    0    0 1534
tail(h4)
##        [,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8]
## [112,]  874 1236    3    4    0    0    0  707
## [113,]  872 1229    6    3    0    0    0  721
## [114,]  870 1273    4    4    0    0    0  638
## [115,]  869 1165    7    4    0    0    0  694
## [116,]  766 1200    7    4    0    0    1  634
## [117,]  739  970    4    4    0    0    1  541
var.names.line <- the.data.line+2
print(h0[var.names.line])
## [1] "PRICE  SQFT  AGE FEATS   NE    CUST     COR         TAX"
var.names <- strsplit(h0[var.names.line]," ")
print(var.names)
## [[1]]
##  [1] "PRICE" ""      "SQFT"  ""      "AGE"   "FEATS" ""      ""     
##  [9] "NE"    ""      ""      ""      "CUST"  ""      ""      ""     
## [17] ""      "COR"   ""      ""      ""      ""      ""      ""     
## [25] ""      ""      "TAX"
var.names <- unlist(var.names)
print(var.names)
##  [1] "PRICE" ""      "SQFT"  ""      "AGE"   "FEATS" ""      ""     
##  [9] "NE"    ""      ""      ""      "CUST"  ""      ""      ""     
## [17] ""      "COR"   ""      ""      ""      ""      ""      ""     
## [25] ""      ""      "TAX"
var.names <- var.names[nchar(var.names)>0]
print(var.names)
## [1] "PRICE" "SQFT"  "AGE"   "FEATS" "NE"    "CUST"  "COR"   "TAX"
var.names <- tolower(var.names)
print(var.names)
## [1] "price" "sqft"  "age"   "feats" "ne"    "cust"  "cor"   "tax"
dimnames(h4)[[2]] <- var.names
head(h4)
##      price sqft age feats ne cust cor  tax
## [1,]  2050 2650  13     7  1    1   0 1639
## [2,]  2080 2600  NA     4  1    1   0 1088
## [3,]  2150 2664   6     5  1    1   0 1193
## [4,]  2150 2921   3     6  1    1   0 1635
## [5,]  1999 2580   4     4  1    1   0 1732
## [6,]  1900 2580   4     4  1    0   0 1534
tail(h4)
##        price sqft age feats ne cust cor tax
## [112,]   874 1236   3     4  0    0   0 707
## [113,]   872 1229   6     3  0    0   0 721
## [114,]   870 1273   4     4  0    0   0 638
## [115,]   869 1165   7     4  0    0   0 694
## [116,]   766 1200   7     4  0    0   1 634
## [117,]   739  970   4     4  0    0   1 541
h5 <- data.frame(h4)
head(h5)
##   price sqft age feats ne cust cor  tax
## 1  2050 2650  13     7  1    1   0 1639
## 2  2080 2600  NA     4  1    1   0 1088
## 3  2150 2664   6     5  1    1   0 1193
## 4  2150 2921   3     6  1    1   0 1635
## 5  1999 2580   4     4  1    1   0 1732
## 6  1900 2580   4     4  1    0   0 1534
tail(h5)
##     price sqft age feats ne cust cor tax
## 112   874 1236   3     4  0    0   0 707
## 113   872 1229   6     3  0    0   0 721
## 114   870 1273   4     4  0    0   0 638
## 115   869 1165   7     4  0    0   0 694
## 116   766 1200   7     4  0    0   1 634
## 117   739  970   4     4  0    0   1 541

The best graphical summary of two continuous variables is a scatterplot. You should include a smooting curve or spline model to the graph to emphasize the general trend and any departures from linearity.

plot(h5sqft,h5$price)
lines(lowess(h5price~h5$sqft))

plot(h5age,h5$price)
sb <- is.finite(h5age)
lines(lowess(h5price[sb]~h5$age[sb]))

The best numeric summary of two continuous variables is a correlation coefficient.

cor(h5[,c("price","sqft","age")],use="pairwise.complete.obs")
##            price        sqft         age
## price  1.0000000  0.84479510 -0.16867888
## sqft   0.8447951  1.00000000 -0.03965489
## age   -0.1686789 -0.03965489  1.00000000

Correlations should always be rounded to two or maybe even just one significant digit.

round(cor(h5[,c("price","sqft","age")],use="pairwise.complete.obs"),1)
##       price sqft  age
## price   1.0  0.8 -0.2
## sqft    0.8  1.0  0.0
## age    -0.2  0.0  1.0

Anyhting larger than 0.7 or smaller than -0.7 is a strong linear relationship. Anything between 0.3 and 0.7 or between -0.3 and -0.7 is a weak linear relationship. Anything between -0.3 and 0.3 represents little or no linear relationship.

The best graphical summary between a continuous variable and a categorical variable is a boxplot.

boxplot(h5price~h5$feats)

boxplot(h5price~h5$ne)

boxplot(h5price~h5$cust)

boxplot(h5price~h5$cor)

If your categorical variable is binary, you can also use a scatterplot. The binary variable goes on the y axis and a trend line is critical.

plot(h5price,h5$ne)
lines(lowess(h5ne~h5$price))

plot(h5price,h5$cust)
lines(lowess(h5cust~h5$price))

plot(h5price,h5$cor)
lines(lowess(h5cor~h5$price))

You can also compute a correlaton between a binary variable and a categorical variable. It is equivalent to the point-bisearial correlation.

round(cor(h5[,c("ne","cust","cor")],h5price),1)
##      [,1]
## ne    0.2
## cust  0.6
## cor  -0.1

Let’s save the display of a relationship involving two categorical variables until another day.

This Blog post was added to the website on 2015-04-03 and was last modified on 2020-02-29. You can find similar pages at R software.

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