dnd 5e – Analyzing a homebrew casting mechanism

Let $p_i$ be the probability of the $i$-th check succeeding, and let $N$ be a random variable describing the number of successful casting attempts. Then the probability of successfully casting the spell at least $k$ times is: $$mathrm P(N ≥ k) = p_1 times p_2 times cdots times p_k = prod_{i=1}^k p_i,$$ where the $Pi$ symbol denotes multiplying together $p_i$ for values of all $i$ from $1$ to $k$.

Knowing these probabilities for all $k$ is enough to fully characterize the distribution of $N$. In particular, it lets us calculate the probability of successfully casting the spell exactly $k$ times: $$mathrm P(N = k) = mathrm P(N ≥ k) – mathrm P(N ≥ k+1) = (1-p_{k+1}) prod_{i=1}^k p_i.$$ (This should make sense when you think about it: it’s just the probability of succeeding on the first $k$ checks and the failing the next one.)

In particular, we can also calculate the expected number of successful casts using either of the following formulas: $$begin{aligned} mathbb E(N) &= sum_{k=1}^infty k times mathrm P(N = k) \ mathbb E(N) &= sum_{k=1}^infty P(N ≥ k) end{aligned}$$

These formulas can be fairly easily applied by hand: first calculate a table of the single-roll success probabilities $p_i$, keeping in mind the changing die size. Then compile another table of the cumulative probabilities $mathrm P(N ≥ k)$ of succeeding on the first $k$ rolls (i.e. the product of the first $k$ items in the first table), and finally sum the numbers in the second table to obtain the expectation value $mathbb E(N)$.

Or you can just make a spreadsheet to do it for you.

Google Sheets screenshot

(Note: the linked spreadsheet is read only. To test it for different spell levels, make a copy first.)

Alternatively, we can calculate the probabilities “backwards” using the following recurrence: $$begin{aligned} N_{k-1} &= begin{cases} 1 + N_k & text{with probability }p_k \ 0 & text{with probability }1-p_k end{cases} \ &= mathbf 1_{p_k} (1 + N_k),end{aligned}$$ where $N_k$ is a random variable denoting the number of successful casting attempts after $k$ successes, and $mathbf 1_{p_k}$ is a random variable that equals $1$ with probability $p_k$ and $0$ otherwise.

Using this formula, it’s quite easy to calculate the distribution of $N$ iteratively using AnyDice and plot the results:

DICE: {20, 12, 10, 8, 6, 4:15}

loop LEVEL over {1..9} {
  loop X over (reverse DICE) {
    SPELLS: (dX > 2*LEVEL) * (1 + SPELLS)
  output SPELLS named "spell level (LEVEL)"

AnyDice screenshot

Of course, to apply this “backwards” formula, we have to start with an approximation of $N_k$ for some sufficiently large value of $k$. The code above effectively assumes that the player will never succeed on more than fifteen d4 rolls (i.e. no more than 20 rolls total), and thus starts with $N_{20} = 0$. (You can change this limit by changing the length of the DICE array.)

For spell level 2 and above this assumption is exact (and in fact overkill), since a d4 roll is guaranteed to fail; for level 1 spells it’s not, but in practice the probability of succeeding on 15 successive checks with a 50% success rate each is negligible ($2^{-15} ≈ 0.00003$), so it’s a very good approximation.