Example 1

Read the Osteoporosis dataset and turn factors into variables automatically with Rbase function read.delim
Take a sample of size 100 from the original file. Call it ‘osteo100’ and work with this file from now on.
Compute the mean value of the variable containing bone density values BUA
Split the computation between all subgroups from variable classific and variable menop
Compute the percentage of menopausic women from variable menop

library(dplyr)

## 
## Attaching package: 'dplyr'

## The following objects are masked from 'package:stats':
## 
##     filter, lag

## The following objects are masked from 'package:base':
## 
##     intersect, setdiff, setequal, union

# Read data
osteoporosis <- read.delim2("datasets/osteoporosis.csv", stringsAsFactors=TRUE)
# Take subsample
osteo100 <- sample_n(osteoporosis, 100)
# mean bone density
buaMean <- mean(osteo100$bua)
print(buaMean)

## [1] 72.9

# Mean bone density ny groups
osteo100 %>% 
  group_by(menop) %>% 
  summarize(m = mean(bua))

# Proportion of menop women (Proportion  is a mean of 0-1 values)
mean(ifelse(osteo100$menop=="SI",1,0))

## [1] 0.8

Exercise 1

Read the diabetes dataset. Convert characters into factors before continuing.
Provide an estimate of
- The distribution of a numerical variable.
- a proportion of at least one categorical variable and
- the mean value of at least one numerical variable.
Could you have used different estimators?
How would you decide?

First we read data and recode character values into factors.

library(readxl)
library(dplyr)
library(magrittr)
diabetes <- read_excel("datasets/diabetes.xls")
sapply(diabetes, class)

##    numpacie        mort    tempsviu        edat         bmi    edatdiag 
##   "numeric" "character"   "numeric"   "numeric"   "numeric"   "numeric" 
##       tabac         sbp         dbp         ecg         chd 
## "character"   "numeric"   "numeric" "character" "character"

diabetes_factor <- diabetes %>%
  mutate_if(sapply(diabetes, is.character), as.factor) %>%
  select (-numpacie)
sapply(diabetes_factor, class)

##      mort  tempsviu      edat       bmi  edatdiag     tabac       sbp       dbp 
##  "factor" "numeric" "numeric" "numeric" "numeric"  "factor" "numeric" "numeric" 
##       ecg       chd 
##  "factor"  "factor"

Next provide a quick summary of each variable

summary(diabetes_factor)

##      mort        tempsviu          edat            bmi           edatdiag    
##  Muerto: 25   Min.   : 0.00   Min.   :31.00   Min.   :18.20   Min.   :26.00  
##  Vivo  :124   1st Qu.: 7.30   1st Qu.:43.00   1st Qu.:26.60   1st Qu.:38.00  
##               Median :11.60   Median :50.00   Median :31.20   Median :45.00  
##               Mean   :10.52   Mean   :52.17   Mean   :31.78   Mean   :45.99  
##               3rd Qu.:13.90   3rd Qu.:60.00   3rd Qu.:35.20   3rd Qu.:53.00  
##               Max.   :16.90   Max.   :86.00   Max.   :59.70   Max.   :81.00  
##         tabac         sbp             dbp               ecg      chd    
##  Ex fumador:41   Min.   : 98.0   Min.   : 58.00   Anormal : 11   No:99  
##  Fumador   :51   1st Qu.:124.0   1st Qu.: 74.00   Frontera: 27   Si:50  
##  No fumador:57   Median :138.0   Median : 80.00   Normal  :111          
##                  Mean   :139.1   Mean   : 90.04                         
##                  3rd Qu.:152.0   3rd Qu.: 88.00                         
##                  Max.   :222.0   Max.   :862.00

Plotting all variables with an instruction is a bit tricky. May be easier to plot separately numerical and categorical variables.

library(ggplot2)
library(tidyr)

## 
## Attaching package: 'tidyr'

## The following object is masked from 'package:magrittr':
## 
##     extract

library(purrr)

## 
## Attaching package: 'purrr'

## The following object is masked from 'package:magrittr':
## 
##     set_names

diabetes %>%
  keep(is.numeric) %>% 
  gather() %>% 
  ggplot(aes(value)) +
    facet_wrap(~ key, scales = "free") +
    geom_histogram()

## `stat_bin()` using `bins = 30`. Pick better value with `binwidth`.

Proceed similarly with categorical variables

diabetes %>%
  keep(is.character) %>% 
  gather() %>% 
  ggplot(aes(value)) +
    facet_wrap(~ key, scales = "free") +
    geom_bar()

You may notice -or not- that the dataset has some outlier values.

Before removing them consider estimating the mean nvalue of SBP and DBP with distinct estimators

with(diabetes_factor, {
    print("DBP")
    show(summary(dbp))
    print("SBP")
    show(summary(sbp))
  }
  )

## [1] "DBP"
##    Min. 1st Qu.  Median    Mean 3rd Qu.    Max. 
##   58.00   74.00   80.00   90.04   88.00  862.00 
## [1] "SBP"
##    Min. 1st Qu.  Median    Mean 3rd Qu.    Max. 
##    98.0   124.0   138.0   139.1   152.0   222.0

What is prefereable to estimate the mean SBP or DBP?

Example 2. Computing Confidence Intervals with R (2)

t.test(osteo100[["bua"]])

## 
##  One Sample t-test
## 
## data:  osteo100[["bua"]]
## t = 40.765, df = 99, p-value < 2.2e-16
## alternative hypothesis: true mean is not equal to 0
## 95 percent confidence interval:
##  69.35159 76.44841
## sample estimates:
## mean of x 
##      72.9

Example 2 . Computing Confidence Intervals with R (3)

cntMenop <- table(osteo100[["menop"]])["SI"]
ssize <- length(osteo100[["menop"]])
prop.test (x=cntMenop, n=ssize)

## 
##  1-sample proportions test with continuity correction
## 
## data:  cntMenop out of ssize, null probability 0.5
## X-squared = 34.81, df = 1, p-value = 3.635e-09
## alternative hypothesis: true p is not equal to 0.5
## 95 percent confidence interval:
##  0.7056770 0.8707518
## sample estimates:
##   p 
## 0.8

Exercise 2.1 Computing Confidence intervals

Read the file “osteoporosis.csv” into a dataset and call it “osteoporosis”
Compute confidence intervals for the BUA mean and for the percentage of menopausic women with all the individuals in the dataset.
Compare these confidence intervals with those that you obtained in example 2. How do they differ?

La solución en el tema 6

Exercise 2.2 Computing Confidence intervals

Read the diabetes dataset. Convert characters into factors before continuing.

library(readxl)
library(dplyr)
library(magrittr)
diabetes <- read_excel("datasets/diabetes.xls")
sapply(diabetes, class)

##    numpacie        mort    tempsviu        edat         bmi    edatdiag 
##   "numeric" "character"   "numeric"   "numeric"   "numeric"   "numeric" 
##       tabac         sbp         dbp         ecg         chd 
## "character"   "numeric"   "numeric" "character" "character"

diabetes_factor <- diabetes %>%
  mutate_if(sapply(diabetes, is.character), as.factor) %>%
  select (-numpacie)
sapply(diabetes_factor, class)

##      mort  tempsviu      edat       bmi  edatdiag     tabac       sbp       dbp 
##  "factor" "numeric" "numeric" "numeric" "numeric"  "factor" "numeric" "numeric" 
##       ecg       chd 
##  "factor"  "factor"

Provide a confidence interval for:
- a proportion of at least one categorical variable and
- the mean value of at least one numerical variable.

cnt <- table(diabetes[["mort"]])["Muerto"]
ssize <- length(diabetes[["mort"]])
prop.test (x=cnt, n=ssize)

## 
##  1-sample proportions test with continuity correction
## 
## data:  cnt out of ssize, null probability 0.5
## X-squared = 64.456, df = 1, p-value = 9.869e-16
## alternative hypothesis: true p is not equal to 0.5
## 95 percent confidence interval:
##  0.1134978 0.2396854
## sample estimates:
##         p 
## 0.1677852

t.test(diabetes[["edat"]])

## 
##  One Sample t-test
## 
## data:  diabetes[["edat"]]
## t = 54.09, df = 148, p-value < 2.2e-16
## alternative hypothesis: true mean is not equal to 0
## 95 percent confidence interval:
##  50.26188 54.07370
## sample estimates:
## mean of x 
##  52.16779

How would you find alternative approaches to compute these confidence intervals?

Using formulas directly calculanting from \(\bar{x}\pm t_{\alpha/2}\frac{s}{\sqrt{n}}\)

Why would you want to do such a thing?

mostra<-diabetes[["edat"]]

m<-mean(mostra)  # Calculate mean
 
sd<-sd(mostra)  # Calculate standard deviation 
se<-sd/sqrt(length(mostra)) # Calculate standard Error
li<- m-qt(.995,length(mostra)-1)*se # Calculate 95% CI lower bound

ls<- m+qt(.995,length(mostra)-1)*se # Calculate 95%CI upper bound


cat("Mean=",m,"\n")

## Mean= 52.16779

cat("Standard deviation=",sd,"\n")

## Standard deviation= 11.77285

cat("Standard error=",se,"\n")

## Standard error= 0.9644696

cat("95% Confidence interval=(",li,";",ls,")","\n")

## 95% Confidence interval=( 49.65104 ; 54.68453 )

Se calcula el intervalo de confianza como \(\overline{p}\pm z_{1-\alpha/2}\sqrt{\frac{\overline{p}(1-\overline{p})}{n}}\)

cnt <- table(diabetes[["mort"]])["Muerto"]
ssize <- length(diabetes[["mort"]])
p<-cnt/ssize
n<-ssize 
z<-qnorm(.975)
ee<-sqrt((p*(1-p))/n)
lowerli<- p-z*ee
upperli<- p+z*ee

cat("95% confidence interval for  ", p ,"=(",lowerli,";",upperli,")","\n")

## 95% confidence interval for   0.1677852 =( 0.1077855 ; 0.227785 )

Example 3. Sample size calculation

Using the osteoporosis dataset, assume that the standard deviation is a good aproximation to \(\sigma\).
Find the sample size needed to achieve a margin of error equal to 5 with a \(95\%\) confidence interval.

Exercise 3. Sample size calculation

Sample size formula is

\[n= \frac{\overline{p}(1-\overline{p})z_{1-\alpha/2}^2} {marginerror^2}\] - Write a function to compute the sample size for proportions in the worst case (p=q=0.5) or assuming \(p\) is known.

Using a \(50\%\) planned proportion estimate, find the sample size needed to achieve \(5%\) margin of error for a survey at \(95%\) confidence level.
How would this result change if we are told that a pilot study suggests that \(p=10\%\)?

alpha<-1-.95
z<-qnorm(1-alpha/2)
merror<-0.05
p<-0.5 # Worst proportion 
nsample<- (p*(1-p) * z^2)/0.05^2
cat("Sample size for 95% CI and 5% margin error s ",round(nsample))

## Sample size for 95% CI and 5% margin error s  384

p<-.1 # Worst proportion 
nsample<- (p*(1-p) * z^2)/0.05^2
cat("Sample size for 95% CI and 10% margin error s ",round(nsample))

## Sample size for 95% CI and 10% margin error s  138

Session 5. Introduction to statistical inference