Explain like I'm five · SW Eng

What is a Function Point (FP)?

Function Points, told as a story about a standard ruler that measures a manuscript by what it does, not by its page count.

so how many pages is this book?manuscript A — 300 pagesmanuscript B — 80 pagespage count alone doesn't say how much work it is

Page count alone doesn't tell you the size.Page count alone doesn't say how much work it is.

pictures just padded the page countdense text means more workpage count alone can't fairly measure the work

Page count isn't a fair ruler.Page count alone can't fairly measure the amount of work.

Function Point = a standard ruler that counts what a manuscript doescount functions, not pagessize is measured the same way, no matter the writer's style

A Function Point is a standard ruler that counts what a manuscript does (its functions), not its pages.Size is measured the same way, no matter the writer's style.

Count the inputshow many functions write data in
Count outputs and lookupshow many functions show results
Count the storagehow many data groups the book keeps
Count the links outhow many data groups other publishers keep, read-only
HOW FP IS CALCULATED① count input/output/inquiry/storage/link② multiply each by a complexity weight③ sum = UFP (unadjusted FP)④ × VAF (adjustment factor) = FP (optional step)FP = UFP × VAFthis score is how you gauge the manuscript's size

FP counts input, output, inquiry, storage, and interface functions, then applies weights.Count × weight gives UFP; multiplying by the adjustment factor gives the final FP. In current IFPUG practice the adjustment is optional, so UFP can stand as the size.

we've measured the manuscript's sizenow we gauge cost and schedule from that sizemeasuring with the ruler is only the starteven at the same size, writing speed varies by writer skill and tools

Measuring with the ruler is only the start.Even at the same size, writing speed varies by writer skill and tools. Next: turning that size into money — cost estimation.

In one breath

Function Point (FP) = a standard ruler that measures a manuscript by counting inputs, outputs, inquiries, storage, and links — not pages.

Function Point (FP) is a sizing technique that counts five function types — external input (EI), external output (EO), external inquiry (EQ), internal logical file (ILF), and external interface file (EIF) — multiplies each by a complexity weight to get the unadjusted FP (UFP), then, if wanted, multiplies by a value adjustment factor (VAF) for the final FP (the adjustment has been optional since IFPUG 4.3). Unlike lines of code (LOC), it's less sensitive to language or authoring style.

When grown-ups say it

Function Point
The ruler that counts a manuscript by function. Measures size by what it does, not by page count.
Unadjusted FP
The sum after weights, before adjustment. The raw score, before adjustment.
Value Adjustment Factor
The multiplier that reflects system characteristics. Computed from 14 traits such as performance and online update, ranging from 0.65 to 1.35.
External Input
A function that writes data into the manuscript. A function that brings data in.
External Output
The result shown to the reader. Data that's processed before it goes out.
External Inquiry
Just looking something up and showing it. A lookup shown as-is, with no processing.
Internal Logical File
A data group the book manages internally. Maintained directly by this publishing house.
External Interface File
Referencing data another publisher maintains. How that connection works → Open API.
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