erroneous datawhat if there' software bug somewhere in some system that' just writing out the wrong values in some set of situationsit can happen unfortunatelythere' no good way for you to know about that butif you see data that just looks fishy or the results don' make sense to youdigging in deeply enough can sometimes uncover an underlying bug that' causing the wrong data to be written in the first place maybe things aren' being combined properly at some point maybe sessions aren' being held throughout the entire session people might be dropping their session id and getting new session ids as they go through websitefor example irrelevant dataa very simple one here maybe you're only interested in data from new york city peopleor something for some reason in that case all the data from people from the rest of the world is irrelevant to what you're trying to find out the first thing you want to do is just throw all that data that away and restrict your datawhittle it down to the data that you actually care about inconsistent datathis is huge problem for examplein addressespeople can write the same address in many different waysthey might abbreviate street or they might not abbreviate streetthey might not put street at the end of the street name at all they might combine lines together in different waysthey might spell things differentlythey might use zip code in the us or zip plus code in the usthey might have country on itthey might not have country on it you need to somehow figure out what are the variations that you see and how can you normalize them all together maybe ' looking at data about movies movie might have different names in different countriesor book might have different names in different countriesbut they mean the same thing soyou need to look out for these things where you need to normalize your datawhere the same data can be represented in many different waysand you need to combine them together in order to get the correct results formattingthis can also be an issuethings can be inconsistently formatted take the example of datesin the us we always do monthdayyear (mm/dd/yy)but in other countries they might do daymonthyear (dd/mm/yy)who knows you need to be aware of these formatting differences maybe phone numbers have parentheses around the area codemaybe they don'tmaybe they have dashes between each section of the numbersmaybe they don'tmaybe social security numbers have dashesmaybe they don' these are all things that you need to watch out forand you need to make sure that variations in formatting don' get treated as different entitiesor different classifications during your processing