The similarities between Daryl Zero and Sir A. C. Doyle's iconic detective Sherlock Holmes have been discussed before, at least here, here, here, and here. In particular, there are striking similarities between the principle thematic elements of Zero Effect and those of "A Scandal in Bohemia," the third of the Holmes stories, featuring the sole romantic entanglement of Holmes' career, and a minimal one at that. Likewise, Daryl Zero experiences the sole (seemingly) romantic entanglement of his career, though, admittedly, one much more explicit and serious in character, with Gloria Sullivan.
The similarities between "A Scandal in Bohemia" and Zero Effect run deeper than general theme, however; in both stories, blackmail, though perhaps justified, is the crime, the blackmailer's secret is revealed during a false fire alarm, a mutual respect emerges between detective and blackmailer, and they meet only while the detective is in disguise (though cunningly identified by blackmailer). When Zero first meets Gloria, he asks her if she is a paramedic; puzzled, she affirms the claim, then asks him how he knew. Zero replies, "I'm very intuitive." Later, we learn, in a typically Holmesian display of "deduction," that it was from the very distinctive smell of iodine that Zero inferred her prior presence in a hospital or ambulance. Likewise, in "A Scandal in Bohemia," Holmes "deduces" that Watson has returned to medical practice from a similar smell:
As to your practice, if a gentleman walks into my rooms smelling of iodoform, with a black mark of nitrate of silver upon his right forefinger, and a bulge on the right side of his top-hat to show where he has secreted his stethoscope, I must be dull, indeed, if I do not pronounce him to be an active member of the medical profession.
There follows a brief discussion of Holmes' method:
I could not help laughing at the ease with which he explained his process of deduction. "When I hear you give your reasons," I remarked, "the thing always appears to me to be so ridiculously simple that I could easily do it myself, though at each successive instance of your reasoning I am baffled until you explain your process. And yet I believe that my eyes are as good as yours."
"Quite so," he answered, lighting a cigarette, and throwing himself down into an armchair. "You see, but you do not observe. The distinction is clear.
We see here two acknowledged cornerstones of Holmes' method: "deduction," and observation. Zero likewise characterizes his method with two principles, the "two obs": observation, and objectivity. Before analyzing the method of Daryl Zero, we would do well to consider Holmes' notion of "deduction."
Typically, a deduction is a truth-preserving procedure, reasoning from true premises to true conclusions. Holmes' "deductions," however, are not truth-preserving; there are many potential explanations for Watson's condition other than his returning to medical practice, perhaps he'd been in another doctor's office and spilled "iodoform" on himself, for instance. Holmes is actually engaging in abductive inference, reasoning backwards from effects to causes. In gross terms, we may even want to call such an inference a "shrewd guess."
Daryl Zero likewise engages in the "shrewd guess" of abductive inference, yet he does not emphasize the inferential step in his method, the "deductive" step, as Holmes would call it. Instead, he emphasizes objectivity, the process by which the relevant facts are sorted from the non-relevant facts. Given that virtually any of the infinitude of facts involved in fully characterizing an event could potentially be relevant to determining the cause of that event, the process of sorting through this infinitude, of zeroing in on that which is relevant, is every bit as crucial to the detective's job as the inferential step.
For example, clever as Zero's inference was from the smell of iodine to Gloria's job as paramedic, it would not have been possible if he had not first identified that smell as a potential clue to her profession (as opposed to her clothes, or demeanor, or the time of day, or any one of an infinitude of factors).
Fluctuations in Zero's ability to remain objective in the selection of pertinent facts correlate with fluctuations in his physical and mental state. When first we encounter Zero, for example, he has been awake for three days, binging on amphetamines. Upon hearing that Gregory Stark is the subject of blackmail, he begins to ruminate upon the fact that "Gregory Stark is the son of a fat man." Zero confirms a few minutes later, as he passes out, that his "abilities are seriously impaired, perhaps even disabled," acknowledging the effect of exhaustion on his ability to objectively sift through the facts and identify those of potential relevance. As it turns out, the obesity of Stark's father is irrelevant to the case.
Perhaps even more interestingly, we can trace the progression of Zero's romantic interest in Gloria through the lapses in his objectivity. Under working conditions, Zero displays an almost preternatural poise, an uncanny ability to guise himself in the garb of mystery (the ingenious Sergio Knight escapade being only one example), yet in two interactions with Gloria he missteps, failing to perceive a relevant fact, losing his objectivity. The first slip occurs when Gloria asks him if he's in town for "that conference"; reluctantly, Zero agrees. "So you're an accountant?" she asks. He's forced to agree again. However, this encounter takes place after his initial meeting with Gloria, the infamous "are you a paramedic" incident (evidence of slippage on Zero's part itself, needless showing off that could potentially jeopardize his cover (though in this case, perhaps ultimately constructive)), immediately after which the talkative gym receptionist informs him that Gloria is "not married." Zero should have realized that the receptionist's laxity in releasing personal information in the furtherance of romance would make any personal information he had given her (including his profession) potentially available to Gloria Sullivan, a woman already romantically linked with him in the receptionist's mind, as evidenced by the nature of her gossip. If Zero had remained objective, rather than succumbing to distraction in the presence of Gloria's charms, he would have realized that exposing himself to being tagged with a career other than architect, by admitting he was in town for "that conference," in the face of the receptionist's tendency to gossip, would have likewise exposed him to discovery.
Of course, only a preternatural talent such as Zero's could possibly have noticed the relevance between the receptionist's comment and Gloria's question, but it is precisely because he does display such deftness elsewhere that our attention is drawn to this slip. Zero's second slip is less excusable; he fails to take the receipt (after claiming receipt-keeping was "just a matter of habit" for him) as the two leave a restaurant on a date (the very date during which we learn both that Gloria is Stark's daughter and that (possibly?) Zero shares a particular family dysfunction with her - he claims that his father killed his mother, then slit his wrists. Gloria's father, Stark, killed her mother, albeit via an intermediary hitman. Zero looks exposed, honest, as he makes this confession; could this be a legitimate point of emotional contact between them? Or, is he just that good? Has he crafted this story, and delivered it convincingly, as a ploy to trigger her emotions? ~ the monologue indeed comes only after Zero has realized who Gloria is . . . ). 
These two slips are noticed by Gloria and provide her with the essential clues to discover Daryl Zero's identity.
Of course, Zero's method must also include an inferential step akin to Holmes' "deduction," but he discusses it, however, only under the description "research":
I can't possibly overstate the importance of good research. Everyone goes through life dropping crumbs; if you can recognize the crumbs, you can trace a path all the way back from your death certificate to the dinner and a movie that resulted in you in the first place. But research is an art, not a science, because anyone who knows what they're doing can find the crumbs—the wheres, whats, and whos—the art is in the whys, the ability to read between the crumbs . . . for every event there is a cause and effect, for every crime a motive, and for every motive a passion. The art of research is the ability to look at the details and see the passion.
The connection here with reasoning backwards from effects to causes is obvious. Yet we also see an emphasis upon analysis of human character and motive. Regardless, the process being described is clearly more inferential than mere "research."
The claim that research is an "art" may at first seem puzzling; for is not the creativity of art somehow at odds with the cold detachment necessary for objectivity? Yet the two are actually entangled, for the exacting standards of objectivity and the true selflessness of great art reach their ultimate embodiment in one and the same faculty.
(I need hardly mention here Holmes' violin or Zero's guitar!)
A similar point was expressed far more eloquently than I ever could by E. A. Poe concerning the relationship between the analytic and the imaginative in the character of the first serialized detective, the enigmatic Auguste Dupin. I leave you with his words:
The analytic power should not be confounded with simple ingenuity; for while the analyst is necessarily ingenious, the ingenious man is often remarkably incapable of analysis. The constructive or combining power, by which ingenuity is usually manifested, and to which the phrenologists (I believe erroneously) have assigned a separate organ, supposing it a primitive faculty, has been so frequently seen in those whose intellect bordered otherwise upon idiocy, as to have attracted general observation among writers on morals. Between ingenuity and the analytic ability there exists a difference far greater, indeed, than that between the fancy and the imagination, but of a character very strictly analogous. It will be found, in fact, that the ingenious are always fanciful, and the truly imaginative never otherwise than analytic.
