Confused around prop.test() and chisq.test()

General
1

Hello everyone:

First of all, I beg you to excuse my lack of statistical knowledge. I am just a "basic user guy" trying to learn R software. Thus, some of my explanations might be unnacurate.

In order to learn about contrast of proportions I tried the following code (I used iris database for anyone to be able to replicate it easily and accurately):

data <- iris
data$Length <- ifelse(data$Petal.Length > 3, "Long", "Short")
data$Width <- ifelse(data$Petal.Width > 1, "Large", "Small")
data$Sepal <- ifelse(data$Sepal.Length > 6, "Big",
                     ifelse(data$Sepal.Length < 5, "Small", "Medium"))
View(data)

table_1x2 <- table(data$Length)
table_1x2
prop.table(table_1x2)
prop.test(table_1x2)$p.value
chisq.test(table_1x2)$p.value

table_2x2 <- table(data$Length, data$Width)
table_2x2
prop.table(table_2x2)
prop.test(table_2x2)$p.value
chisq.test(table_2x2)$p.value

table_3x2 <- table(data$Species, data$Length)
table_3x2
prop.table(table_3x2)
prop.test(table_3x2)$p.value
chisq.test(table_3x2)$p.value

table_3x3 <- table(data$Species, data$Sepal)
table_3x3
prop.table(table_3x3)
prop.test(table_3x3)$p.value
chisq.test(table_3x3)$p.value

The result of all test are the same with two exceptions: table_3x3 and table_1x2. I thought that both test might be equivalents (although prop.test would not be suitable in 3x3 or larger tables) but I don't get why I am getting different p-values in the 1x2 table.

2

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