dataframe - R: Selecting first of n consecutive rows above a certain threshold value

Question

I have a data frame with MRN, dates, and a test value.

I need to select all the first rows per MRN that have three consecutive values above 0.5.

This is an example version of the data:

   MRN Collected_Date   ANC
1  001     2015-01-02 0.345
2  001     2015-01-03 0.532
3  001     2015-01-04 0.843
4  001     2015-01-05 0.932
5  002     2015-03-03 0.012
6  002     2015-03-05 0.022
7  002     2015-03-06 0.543
8  002     2015-03-07 0.563
9  003     2015-08-02 0.343
10 003     2015-08-03 0.500
11 003     2015-08-04 0.734
12 003     2015-08-05 0.455
13 004     2014-01-02 0.001
14 004     2014-01-03 0.500
15 004     2014-01-04 0.562
16 004     2014-01-05 0.503

Example code:

df <- data.frame(MRN = c('001','001','001','001',
                         '002','002','002','002',
                         '003','003','003','003',
                         '004','004','004','004'), 
                 Collected_Date = as.Date(c('01-02-2015','01-03-2015','01-04-2015','01-05-2015',
                                            '03-03-2015','03-05-2015','03-06-2015','03-07-2015',
                                            '08-02-2015','08-03-2015','08-04-2015','08-05-2015',
                                            '01-02-2014','01-03-2014','01-04-2014','01-05-2014'), 
                                            format = '%m-%d-%Y'), 
                 ANC = as.numeric(c('0.345','0.532','0.843','0.932',
                         '0.012','0.022','0.543','0.563',
                         '0.343','0.500','0.734','0.455',
                         '0.001','0.500','0.562','0.503')))

Currently, I am using a very awkward approach using the lag function to calculate the date difference, then filter for all values >= 0.5, and then sum up the values, which helps to select the date of the THIRD value. I then substract two days to get the date of the first value:

   df %>% group_by(MRN) %>% 
    mutate(., days_diff = abs(Collected_Date[1] - Collected_Date)) %>% 
        filter(ANC >= 0.5) %>%
            mutate(days = days_diff + lag((days_diff))) %>%
                filter(days == 5) %>%
                    mutate(Collected_Date = Collected_Date - 2) %>%
                        select(MRN, Collected_Date)

Output:

Source: local data frame [2 x 2] Groups: MRN

  MRN Collected_Date
1 001     2015-01-03
2 004     2014-01-03

There must be a way simpler / more elegant way. Also, it does not give accurate results if there are gaps between the test dates.

My desired output for this example is:

   MRN Collected_Date   ANC     
1  001     2015-01-03 0.532
2  004     2014-01-03 0.500

So if at least three consecutive test values are >= 0.5, the date of the FIRST value should be returned.

If there are not at least three consecutive values >= 0.5, NA should be returned.

Any help is greatly appreciated!

Thank you very much!

Answer 1

The easiest way is to use the zoo library in conjunction with dplyr. Within the zoo package there is a function called rollapply, we can use this to calculate a function value for a window of time.

In this example, we could apply the window to calculate the minimum of the next three values, and then apply the logic specified.

df %>% group_by(MRN) %>%
  mutate(ANC=rollapply(ANC, width=3, min, align="left", fill=NA, na.rm=TRUE)) %>%
  filter(ANC >= 0.5) %>%  
  filter(row_number() == 1)

#   MRN Collected_Date   ANC
# 1 001     2015-01-03 0.532
# 2 004     2014-01-03 0.500

---

In the code above we have used rollapply to calculate the minimum of the next 3 items. To see how this works compare the following:

rollapply(1:6, width=3, min, align="left", fill=NA) # [1]  1  2  3  4 NA NA
rollapply(1:6, width=3, min, align="center", fill=NA) # [1] NA  1  2  3  4 NA
rollapply(1:6, width=3, min, align="right", fill=NA) # [1] NA NA  1  2  3  4

So in our example, we have aligned from the left, so it starts from the current location and looks forward to the next 2 values.

Lastly we filter by the appropriate values, and take the first observation of each group.