Those who follow manga sales are probably aware of the not-at-all-uncommon phenomenon where, following an anime that successfully catches people’s attention, every volume of the series, old and new, gets back on the Oricon weekly charts, and sometimes stays there for an extended period of time. The resulting re-chart numbers for the series can be broken down in a number of ways. I’m going to be looking on a particular test case (Nanatsu no Taizai’s performance over last Fall) which shows how those numbers can be interpreted.

The Nanatsu no Taizai manga series got what is by any metric a substantial boost in sales from the anime. From the 13-week period starting with the week of October 6-12 and going until the end of December, the first 10 volumes charted together, taking up one-fifth of the top 50, in 8 separate weeks. However, the way they charted evolved as time went on. Here’s how the volumes of the series stacked up against each other on that first October week:

10. *48,513 *,476,099 Nanatsu no Taizai Vol.1
11. *45,629 *,453,452 Nanatsu no Taizai Vol.2
16. *40,609 *,424,405 Nanatsu no Taizai Vol.3
19. *36,570 *,396,046 Nanatsu no Taizai Vol.4
24. *33,583 *,394,860 Nanatsu no Taizai Vol.5
26. *30,447 *,383,580 Nanatsu no Taizai Vol.6
28. *28,983 *,374,842 Nanatsu no Taizai Vol.7
30. *28,180 *,332,249 Nanatsu no Taizai Vol.10
31. *28,084 *,354,996 Nanatsu no Taizai Vol.9
32. *27,927 *,369,899 Nanatsu no Taizai Vol.8

And here’s what they looked like at the end of December:

25.　*37,085　*,667,255　Nanatsu no Taizai Vol.10
31.　*33,864　*,700,757　Nanatsu no Taizai Vol.7
32.　*33,703　*,671,099　Nanatsu no Taizai Vol.9
33.　*33,516　*,689,629　Nanatsu no Taizai Vol.8
36.　*31,451　*,706,396　Nanatsu no Taizai Vol.6
39.　*28,361　*,721,921　Nanatsu no Taizai Vol.4
40.　*28,122　*,718,480　Nanatsu no Taizai Vol.5
41.　*27,717　*,811,313　Nanatsu no Taizai Vol.1
42.　*27,659　*,782,681　Nanatsu no Taizai Vol.2
43.　*27,597　*,749,448　Nanatsu no Taizai Vol.3

Quite a difference in order there. That first week saw the volumes selling basically by volume number priority (v8-10 were out of order, but separated by less than 300 units-effectively tied), but 13 weeks later that trend was reversed, with the latter half of the series’ run on top by several thousand units. This difference is caused in large part by a simple factor – not everybody who buys the manga as a result of the anime buys the whole thing at once. Either because manga fans are either young and yen-strapped or hesitant to buy a whole manga based on one or two episodes of anime, they often start off by buying the first volume or first few volumes. One of the quickest ways to measure that effect is to look at the lifetime totals of the early volumes compare with that of the most recent volume. If there’s a difference in the speed at which the volumes gain sales, it should be fairly obvious there, as early volumes separate and then “rebound” as the later volumes catch up. I took the overall sales figures for v1-v10 each week, using weekly sales to derive those numbers for weeks which had a gap, and produced the following plot:

(Note: -1 on the x-axis means the week before the anime aired and boosts started happening. I derived this by subtracting weekly sales from Oct 6-12 from the same week’s lifetime totals. It’s essentially the pre-anime state of the series.)

This chart, I think, makes it fairly obvious that early volumes create a gap between themselves and the later ones in the first month of a boost, then see that gap stay at constant levels for a while before finally shrinking down to close to where it originally was. Meanwhile, the later volumes fail to separate themselves from the most recent one – the gap even shrinks a bit for them! This suggests a possible pattern of group behavior for newer fans of this series:

1. In the first week, new fans buy the early volumes (sometimes v1, sometimes v1-v3) to check things out and see if the manga is hot stuff, or because they lack the pocket money to buy in bulk. Others with less inhibition buy the whole series.

2. Over the next week or two, fans who bought the early volumes who are now sold on the series and/or in possession of their latest allowance/paycheck, buy the rest of the series. Meanwhile, fans who waited an extra episode or two to go in for the manga get into the action, buying the first few volumes as others did a week ago, obscuring the effects of the first group’s delayed purchasing.

3. Over time, the supply of new fans from the anime tapers off and the later volumes become more prominent in the weekly charts.

These data are important because, if nothing else, they show that manga don’t catch on in an instant with many individual buyers – the differences in week-to-week sales between volumes 1 and 10 are too large for Oricon fuzz to account for all of them. Many people aren’t going to buy volume 12 of the series without having first bought 1-11, and it’s demonstrable here that their doing so takes time. This is particularly important to consider when looking at manga boosts for series with double-digit length and a young/teen target demographic, where time-lag of this type would prevent the full magnitude of the boost from being immediately visible in a new volume coming out two days after the anime’s airdate.*

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Of course, this data is just as it pertains to Nanatsu no Taizai. Other series can and do have different sets of fans who likely behave differently. There are other series one could compare to get a better baseline (Shingeki no Kyojin, Tokyo Ghoul, etc.), but the overwhelming fraction of manga, even those that make the Oricon charts, don’t sell enough to get the entire series back on them for an extended period of time. What I’d really like to be able to compare would be a short series targeted more towards adults, but those less frequently get adapted, and even those that do get to re-chart in their entirety do so sporadically. Gekkan Shojo Nozaki-kun, for example, re-charted all volumes for multiple weeks, but only started doing so 2 weeks after the anime started. By that point in Nanatsu no Taizai’s run, the gaps in volume totals had already locked down in size. In theory, shorter series with more adult fans could see less delay between their fans buying the first and last volumes, but there just aren’t a lot of comparable test cases out there.

*I say this in particular because I’ve encountered a decent share of people who diagnosed Akame ga Kill as an anime which produced no manga boost based on data from a volume that came out before the series aired on TV. The series in question went from selling 52,214 in 2 weeks (v10) pre-anime to 168,053 in 2 weeks (v11) post-anime. It’s not the first time such a delayed-impact boost has happened either; Binbougami ga and Toriko both got one. At least with multi-volume teen-audience series, calling it early is likely to mean calling it wrong on a regular basis.