## Definition Give a detailed, accurate account of a situation, event, pattern, or process—stating not just *what* happens, but *why* it happens or *how* one thing causes another. *Verb. Noun form: explanation.* ## Characteristics - Answers _why_, or _how_ in the causal sense; - builds on a [[describe|description]], adding reasons or causes; - requires discussion of cause, reason, or effect; and - can include examples. ## Examples ### Explain the effects of longshore drift - Sediment moves gradually along the coast, which causes beaches to grow in the direction of drift over time; - this can lead to the formation of features such as spits, where the coastline changes direction and sediment continues to be deposited; - sediment is removed from some stretches of coast faster than it's replaced, so these areas are more exposed to erosion; - as a result, human structures near the coast—such as roads or buildings—can be undermined or lost. ### Explain why IPv6 was introduced - IPv4 addresses are 32 bits long, which allows around 4 billion unique addresses; - the number of internet-connected devices has grown beyond this—exceeding the number of available IPv4 addresses; - therefore IPv6 was introduced, using 128-bit addresses; - this provides a far larger address space, so every device can have a unique address. ### Explain how a binary search determines a value doesn't appear in a given array - The search keeps halving the list, discarding the half that cannot contain the target; - this continues until the list is reduced to one item that doesn't match the search term; - since every other item has already been ruled out, the value must be absent from the array. ### Explain why a payroll processing system is amenable to a computational approach The below explanation builds on [[describe#Describe the features that make a payroll processing system solvable by computational methods|this description]] and this [[justify#Justify the features of a payroll processing system that make it solvable by computational methods|justification]]. Taken together, the features of payroll processing point to the same underlying conclusion: this is a problem defined by consistent, rule-based transformation of structured data at scale, which is precisely the kind of problem computers are built to solve well. The volume and repetition mean a solution only needs to be correct once, since the same logic can then be applied automatically to every record—so the value of automating the problem grows with every additional employee processed, rather than staying fixed. The regional variation doesn't complicate this picture so much as confirm it: a computer can hold and apply many conditional rules with perfect consistency in a way that becomes increasingly error-prone for a person doing the same thing by hand across thousands of records. And because the data going in and the output required are both fixed and predictable, there is no ambiguity for a computer to resolve or interpret before it can act—every record can be processed the same way, every time. It is this combination of scale, repeatability, and structure—not any single feature in isolation—that makes payroll processing amenable to a computational approach: it is a problem where doing the same well-defined thing correctly and consistently, over and over, is exactly what is required, and that is exactly what computers do well. ### Explain how a user of a payroll processing system would use it to make their life easier, and what makes it appropriate for their needs One user of a payroll processing system is a small business's HR administrator, who is responsible for running payroll each month and issuing payslips to all employees. She would use the system to enter each employee's hours or salary once, and have the calculations, tax deductions, and payslip generation handled automatically for the entire workforce. This is appropriate to her needs because she is not a specialist in tax law and cannot be expected to manually recalculate deductions for every employee, especially where regional tax rules differ—the system removes the risk of manual calculation errors and the time cost of processing each employee individually, allowing her to complete payroll accurately even as the number of employees grows. ## Non-examples - [[describe|Description]]—gives the steps of a process, without giving reasons why those steps lead to the result.