
Data points are indeed crucial to this episode, as the title hints, though the more overt implication of the title is death: when your number's up, you die. The plane crash referenced above has killed 81 people, and the airline is considering how best to approach compensation for the victims' beneficiaries. They are considering two possibilities. One is very cost/benefit focused, and would probably ultimately save the airline a lot of money: lawyer Arlen Schrader (Michael Cumpsty) has worked out a complex algorithm for calculating the monetary value of a human life, factoring in consideration of socio-economic status, projected lifelong earnings, health factors (e.g. disease, or unhealthy habits such as smoking--Powell discounts the a mount of cash she was going to leave on one body when she learns he is a smiker). This precise measurement of the value of a human life in strictly monetary terms is indeed an unsavoury reality of how corporations assess payout options when their products kill people, and the episode unsurprisingly presents Holmes as contemptuous of Schrader's reductive calculation of human worth. We might recall recent episodes with similar concerns, such as the debt purchase episode ("Hemlock," from two weeks ago), or "The Eternity Injection," from a few weeks earlier, in which a rich man was willing to sacrifice others to find a cure for himself, calculating his own value a signficantly greater than that of anyone else, or even the AI episode, "Bella," in which the potential cost to humanity as a whole should an AI be created is used as a justification for sacrificing humans to prevent such a development. "When your Number's Up" asks the question in considerably more explicit terms than have other episodes: how do you calculate, exactly, what a human life is worth?
Given that Holmes's (and Watson's) methodology is grounded in rational analysis, such a question
has clear resonance for them. Does a purely analytical relationhsip with others constitute an appropriately human interaction? Can everything be calculated? Dana Powell is from this perspective a sort of anti-Holmes, or more precisely, anti-Watson. She is so because the other payout strategy the airline is considering is a flat payout to all the beneficiaries, regardless of what a Schrader-style algorithm would show. Such a payout would mean that the beneficiaries of the richer passengers (generally those in first class) would get considerably less than the 15 million or so it is estimated the algorithm would give them, but that the beneficiaries of the other passengers would each get in the range of 5 million, significantly more than the Schrader numbers would provide. Since the murderer is explicitly not merely eliminating potential beneficiaries but also leaving cash on their bodies and notes drawing attention to the algorithmic evaluation of human life, the killer evidently has some interest in how the financial question will be settled. Team Holmes deduces that the killer is attempting to create enough public outrage at the idea of Schrader-style human economics (the killer is also ensuring that the media gets the inside story on the evidence on the bodies) to drive the airline to adopt the flat rate payment method. The likely suspect, they therefore conclude, must be one of the beneficiaries who would fare poorly under the graduated scale model.
This deduction leads to an evaluation of the suspect pool (who are also the potential victim pool) in terms of who would gain from the flat rate payout. Hence Holmes's misdirected suspect list. Ironically, despite his objection to Schrader's method of calculating human life in mathematical terms, Holmes automatically excludes the beneficiaries of the first-class victims as suspects, since the higher monetary value of those victims means the flat-rate payout would be detrimental rather than beneficial to them. In effect, Holmes is as reductively analytical as Schrader.
More interesting is the contrast between Watson and Powell. Watson just lost Andrew and is trying

For Powell, therefore, all that matters is the money; though we don't see her relationship with her husband, we can be pretty sure that she saw him as a wealthy meal ticket. Since we do see her strained relationship with her own sister, and her willingness to kill that sister when the sister grasps Powell's involvement in the murders, Powell's sociopathic nature is evident, and is focused specifically on her willingness to use calculation and manipulation to benefit herself, regardless of the cost to others. That she has met the victims she kills at a support group for survivors furthers our recognition of the contrast with Watson, whose career as a sober companion makes the use of such groups to help people, rather than to troll for victims, a key component of her character. Watson is rational, well able to calculate and evaluate, but she is the humanizing face of the Holmes/Watson partnership. Powell represents an exaggerated example of what the Holmesian approach to cold analysis might become, lacking a Watsonian influence.
That Watsonian influence is, by the end of the episode, restored to the brownstone, In contrast to Powell's
struggles to hold onto her house, Watson surrenders her apartment. However, she retains some private space in the brownstone, as we get a new (and presumably recurring set): the basement, which Watson claims as her own space. The Holmes/Watson relationship seems set to continue with a somewhat new balance. Watson may be recommitting to detection and skeptical that she can have a normal life, but she is not becoming merely a copy of Holmes--and, we can be fairly sure, she will never become a Dana Powell.
How did you like the episode? Let me know in the comments below!