Date is Key 

When sending reports via SFTP or email, remember that Funnel keys data by date. If two reports cover the same date range, the latest report will overwrite the earlier one. Funnel also reads the minimum and maximum dates from each file, and any missing dates within that range will be treated as null, possibly overwriting previous data. To fix gaps, submit a new file with the correct data.

Example:
Report A. Sent 10/01/2019

Report B. Sent 10/01/2019

The date for Row 1 of Report B contains the same date as Row 3 and Row 4 of Report A. Because Report B was sent most recently, Row 1 of Report B will overwrite Row 3 and Row 4 of Report A.
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The result after sending Report B in Funnel is the following:

When Funnel reads a file it takes a minimum and maximum date within the file. If there is no data associated with a certain date within the range of the file, Funnel will attribute that missing date null values. This mainly poses issues when trying to attach one missing/incorrect data point to the end of a current report.

Example:
Report A Sent 10/01/2019

Report B Sent 10/01/2019

If you, for instance, realize Row 3 within Report A had the wrong amount for Clicks, so you attached the correct data at the end of Report B. The minimum date within Report B is now 10/03/2019, and the maximum date within Report B is 10/08/2019. Because there is no data within Report B for 10/04/2019, Funnel will regard this as null values and overwrite existing data for these dates. 

The result after sending Report B in Funnel is the following:
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If you find gap like the one above you can always send in a new file with the current and missing data.