A notebook records 4 observations on Monday, 3 on Tuesday and 5 on Wednesday. A second column shows 4, 7 and 12. Neither column is necessarily wrong: one records each day, while the other keeps a running total.
Label the accumulation
For this invented example, the total starts at zero immediately before Monday. Tuesday’s cumulative value is 4 + 3 = 7, and Wednesday’s is 7 + 5 = 12.
Calling the second column simply “count” leaves the reader to guess. A label such as “total since Monday morning” explains its starting boundary.
Recover a daily change carefully
Subtract Tuesday’s cumulative total from Wednesday’s: 12 − 7 = 5. That recovers Wednesday’s count because the totals use the same definitions and continuous accumulation.
If the series resets, changes coverage or contains corrections, the difference needs further explanation. Do not assume every fall in a real running total represents negative observations.
Keep the two questions separate
“How many happened today?” and “How many have happened since the start?” can both matter. A chart title should tell readers which one they are seeing.
Use informative labels to attach the period and unit. The calculation is simple; the care lies in preserving what the number actually counts.