most errors of this type are easy to fix with simple assignment statements and if-thenelse rulesif ="godo" "goodif ="bade" "badredundant whitespace whitespaces tend to be hard to detect but cause errors like other redundant characters would who hasn' lost few days in project because of bug that was caused by whitespaces at the end of stringyou ask the program to join two keys and notice that observations are missing from the output file after looking for days through the codeyou finally find the bug then comes the hardest partexplaining the delay to the project stakeholders the cleaning during the etl phase wasn' well executedand keys in one table contained whitespace at the end of string this caused mismatch of keys such as "fr "fr"dropping the observations that couldn' be matched if you know to watch out for themfixing redundant whitespaces is luckily easy enough in most programming languages they all provide string functions that will remove the leading and trailing whitespaces for instancein python you can use the strip(function to remove leading and trailing spaces fixing capital letter mismatches capital letter mismatches are common most programming languages make distinction between "braziland "brazilin this case you can solve the problem by applying function that returns both strings in lowercasesuch as lower(in python "brazillower(="brazillower(should result in true impossible values and sanity checks sanity checks are another valuable type of data check here you check the value against physically or theoretically impossible values such as people taller than meters or someone with an age of years sanity checks can be directly expressed with rulescheck <age < outliers an outlier is an observation that seems to be distant from other observations ormore specificallyone observation that follows different logic or generative process than the other observations the easiest way to find outliers is to use plot or table with the minimum and maximum values an example is shown in figure the plot on the top shows no outlierswhereas the plot on the bottom shows possible outliers on the upper side when normal distribution is expected the normal distributionor gaussian distributionis the most common distribution in natural sciences