---
title: "Stat. 450 Quiz preparation"
output:
  word_document: default
  pdf_document: default
  html_notebook: default
---

**Stat. 450:** Quiz preparation

These question is related to the homework problem 5.6.7 Exercise 1.

For the flights data, in the nycflights13 package, answer the following questions:

```{r message=FALSE}
library(nycflights13)
library(tidyverse)
```

1. How many flights arrived on time (which includes the flights that arrived early)?

To answer this question we will look at the arr_delay to look at the flights that arrived on time.

**Answer:** 194,342

```{r}
flights %>% select(arr_delay) %>%
  filter(arr_delay <= 0) %>%
  summarize( n=n() )
```


2. What proportion of flights arrived on time? 

To answer this question we will look at the arr_delay to look at the flights that arrived on time.

**Answer:** Approximately 60%.

```{r}
flights %>% select(arr_delay) %>%
  summarize( arr_delay_mean = mean(arr_delay <= 0, na.rm = TRUE) )
```

3. How many United flights arrived 30 or more minutes late?

**Answer:** The number of UA flights that arrived 30 or more minutes late was 8131.


```{r}
flights %>% filter( carrier == "UA") %>%
  count(arr_delay >= 30)
```


4. Which airline has the best on-time performance? 

**Answer:** AS

```{r}
flights %>% select(arr_delay, carrier) %>%
  group_by(carrier)  %>%
  summarize( n=n(), arr_delay_mean=mean(arr_delay <= 0, na.rm = TRUE) ) %>%
  arrange(desc(arr_delay_mean))
```

5. Which airline has the worst on-time performance?

**Answer:**  FL

```{r}
flights %>% select(arr_delay, carrier) %>%
  group_by(carrier)  %>%
  summarize( n=n(), arr_delay_mean=mean(arr_delay <= 0, na.rm = TRUE) ) %>%
  arrange(arr_delay_mean)
```