Pandas - Fixing Wrong Data
Wrong Data
"Wrong data" does not have to be "empty cells" or "wrong format", it can just be wrong, like if someone registered "199" instead of "1.99".
Sometimes you can spot wrong data by looking at the data set, because you have an expectation of what it should be.
If you take a look at our data set, you can see that in row 7, the duration is 450, but for all the other rows the duration is between 30 and 60.
It doesn't have to be wrong, but taking in consideration that this is the data set of someone's workout sessions, we conclude with the fact that this person did not work out in 450 minutes.
Duration Date Pulse Maxpulse Calories
0 60 '2020/12/01' 110 130 409.1
1 60 '2020/12/02' 117 145 479.0
2 60 '2020/12/03' 103 135 340.0
3 45 '2020/12/04' 109 175 282.4
4 45 '2020/12/05' 117 148 406.0
5 60 '2020/12/06' 102 127 300.0
6 60 '2020/12/07' 110 136 374.0
7 450 '2020/12/08' 104 134 253.3
8 30 '2020/12/09' 109 133 195.1
9 60 '2020/12/10' 98 124 269.0
10 60 '2020/12/11' 103 147 329.3
11 60 '2020/12/12' 100 120 250.7
12 60 '2020/12/12' 100 120 250.7
13 60 '2020/12/13' 106 128 345.3
14 60 '2020/12/14' 104 132 379.3
15 60 '2020/12/15' 98 123 275.0
16 60 '2020/12/16' 98 120 215.2
17 60 '2020/12/17' 100 120 300.0
18 45 '2020/12/18' 90 112 NaN
19 60 '2020/12/19' 103 123 323.0
20 45 '2020/12/20' 97 125 243.0
21 60 '2020/12/21' 108 131 364.2
22 45 NaN 100 119 282.0
23 60 '2020/12/23' 130 101 300.0
24 45 '2020/12/24' 105 132 246.0
25 60 '2020/12/25' 102 126 334.5
26 60 20201226 100 120 250.0
27 60 '2020/12/27' 92 118 241.0
28 60 '2020/12/28' 103 132 NaN
29 60 '2020/12/29' 100 132 280.0
30 60 '2020/12/30' 102 129 380.3
31 60 '2020/12/31' 92 115 243.0
How can we fix wrong values, like the one for "Duration" in row 7?
Replacing Values
One way to fix wrong values is to replace them with something else.
In our example, it is most likely a typo, and the value should be "45" instead of "450", and we could just insert "45" in row 7:
For small data sets you might be able to replace the wrong data one by one, but not for big data sets.
To replace wrong data for larger data sets you can create some rules, e.g. set some boundaries for legal values, and replace any values that are outside of the boundaries.
Example
Loop through all values in the "Duration" column.
If the value is higher than 120, set it to 120:
if df.loc[x, "Duration"] > 120:
df.loc[x, "Duration"] = 120
Removing Rows
Another way of handling wrong data is to remove the rows that contains wrong data.
This way you do not have to find out what to replace them with, and there is a good chance you do not need them to do your analyses.
Example
Delete rows where "Duration" is higher than 120:
if df.loc[x, "Duration"] > 120:
df.drop(x, inplace = True)