First, a detailed description of function points Then, how to use function points and lines of code for cost estimation.

Transcription

1 Cost Page 1 Cost modeling Monday, October 05, :17 AM First, a detailed description of function points Then, how to use function points and lines of code for cost estimation. Reading: SEPA Chapter 27.

2 Cost Page 2 Function points Monday, October 05, :18 AM First, tabulate the interfaces of your software with the outside world External Inputs (EIs): originate from users or are transmitted from another application. External Outputs (EOs): sent to user or to another application. External Inquiries (EQs): transactions in which an external source requests and receives information about application state. Internal Logical Files (ILFs): logical groupings of data that reside within the application. External Interface Files (EIFs): files external to the application that are used by the application.

3 Cost Page 3 The FP table Monday, October 05, :22 AM weights Information Domain simple avg complex External Inputs External Outputs External Inquiries Internal Logical Files External Interface Files Basic (raw) function points count interfaces multiply by weight add together

4 Cost Page 4 Example 1: ATM Monday, October 05, :27 AM An ATM machine reads a customer' s card number from a credit card. It then asks the customer to type the last four digits of the card number, and compares them with the number read from the card to make sure it read the card correctly. If the numbers match, the ATM machine asks the customer for a PIN code and compares that with a PIN code stored on its central server for the card number. If those match, the customer is given access; otherwise access is denied. External inputs: card number, last four digits, PIN code, response from server External outputs: ask for digits, ask for PIN, inform of access, ask central server whether PIN matches. External inquiries: 0 Internal logical files: 0 External interface files: 0 Function point calculation: 4 * 3 (simple) + 4 * 4 (simple) = 28 function points.

5 Cost Page 5 Example 2: Airplane GPS Monday, October 05, :37 AM An airplane navigational GPS system takes input from up to 12 navigational satellites and outputs a display of current location to the pilot. It overlays satellite maps obtained from locally stored data onto its display. It also provides a different display to passengers, which has three screens (local, flight, and world). It also responds to queries for airplane location from air traffic control using the aircraft's radio system, and sends its location to the flight recorder. External Inputs: 12 (average) External outputs: 6 (complex) External inquiries: 1 (simple) Internal logical files: 1 (complex) External Interface files: 0 Domain Value simple avg complex EIs EOs EQs ILFs EIFs *4+6*7+1*3+1*15 = 108 Function Points

6 Cost Page 6 Adjustment Monday, October 05, :48 AM Adjustment factors Function points describe what the application interacts with. But there is more than just interaction. Solution: scale the FP value according to expectations. Adjustment factor questionnaire (page 621). 1. Does the system require reliable backup and recovery? 2. Are specialized data communications required to transfer information to or from the application? 3. Are there distributed processing functions? 4. Is performance critical? 5. Will the system run in an existing, heavily utilized operational environment? 6. Does the system require online data entry? 7. Does the online data entry require the input transactions to be built over multiple screens or operations? 8. Are the Internal Logical Files (ILFs) updated online? 9. Are the inputs, outputs, files, or inquiries complex? 10. Is the internal processing complex? 11. Is the code designed to be reusable? 12. Are conversion and installation included in the design? 13. Is the system designed for multiple installations in different organizations? 14. Is the application designed to facilitate change and ease of use by the user?

7 Cost Page 7 Example 1 cont'd: adjustment factors Monday, October 05, :57 AM An ATM machine reads a customer' s card number from a credit card. It then asks the customer to type the last four digits of the card number, and compares them with the number read from the card to make sure it read the card correctly. If the numbers match, the ATM machine asks the customer for a PIN code and compares that with a PIN code stored on its central server for the card number. If those match, the customer is given access; otherwise access is denied. Adjustment factor questionaire. 1. Backup and recovery? 2. Specialized data communications? 3. Distributed processing functions? 4. Is performance critical? 5. Heavily utilized environment? 6. Online data entry? 7. Multiple screens or operations? 8. ILFs updated online? 9. Are data complex? 10. Internal processing complex? 11. Designed to be reusable? 12. Conversion and installation included? 13. Multiple installations in different organizations? 14. Change and ease of use by the user? Adjustment factors = 20 Adjusted function points = counted points * ( *sum of factors) = 28*( *20) = 23.8

8 Cost Page 8 Example 2 cont'd Monday, October 05, :57 AM An airplane navigational GPS system takes input from up to 12 navigational satellites and outputs a display of current location to the pilot. It overlays satellite maps obtained from locally stored data onto its display. It also provides a different display to passengers, which has three screens (local, flight, and world). It also responds to queries for airplane location from air traffic control using the aircraft's radio system, and sends its location to the flight recorder. Adjustment factor questionaire. 1. Backup and recovery? 2. Specialized data communications? 3. Distributed processing functions? 4. Is performance critical? 5. Heavily utilized environment? 6. Online data entry? 7. Multiple screens or operations? 8. ILFs updated online? 9. Are data complex? 10. Internal processing complex? 11. Designed to be reusable? 12. Conversion and installation included? 13. Multiple installations in different organizations? 14. Change and ease of use by the user? Adjustment factors = 31 Adjusted function points = counted points * ( *sum of factors) = 108*( *31) =

9 Cost Page 9 From function points to cost Monday, October 05, :20 PM Cost models vary in what their inputs are Some take function points object points lines of code LOC per FP (Page 674) lang median low high C C Note the variation in translation for C!

10 Cost Page 10 Simple cost modeling Monday, October 05, :15 PM If a certain size of project S has taken a P person months, and you have the same size project, then yours will take P person months as well. But is this linear, i.e., can one really say that a project of size 2S takes 2P months? For a million reasons, no!

11 Cost Page 11 Simple cost estimation Monday, October 05, :39 PM Statistical cost estimation Input: FP and cost for several historical projects. Method: fit Effort in person months = A + B * (KLOC) C to historical data for KLOC and effort to determine A, B, C. Computation: plug in measured KLOC, get effort.

12 Cost Page 12 COCOMO-81 Monday, October 05, :50 PM COCOMO: the constructive cost model Proposed by Boehm Has gone through several revisions Basic idea: COCOMO-81 Basic idea of COCOMO-81: (simple estimation) Characterize the software to be written Organic: having few inputs and outputs (e.g., cat). Semi-detached: interacting in an average way with the operating system (e.g., gcc) Embedded: interacting in a sophisticated manner with the operating system (e.g., emacs). Then effort in person months is E = a(kloc) b where a and b are determined by the following table: Project a b Organic Semi-detached Embedded And development time in months is D=c(E) d where c and d are determined by the following table: Project c d Organic Semi-detached Embedded

13 Cost Page 13 Putnam's equation Monday, October 05, :50 PM Some intuition: Putnam's equation LOC * B 1/3 E = P * T 4 where LOC = lines of code B = "special skills factor" P = productivity factor T = project duration in months E = estimated person-months typical values: P=2,000 P=10,000 P=28,000 B=.16 B=.36 embedded systems systems software business systems small programs > 70 KLOC

14 Cost Page 14 COCOMO-II Monday, October 05, :51 PM Generations of CoCoMo COCOMO-I: simple estimates of effort based upon software attributes COCOMO-II: tiered cost estimation based upon Product complexity Project attributes Site attributes That becomes more accurate as the project progresses.

15 Cost Page 15 Inside COCOMO-II Monday, October 05, :54 PM Basic attributes systems engineering cost drivers: things that make designing the system more difficult. software scale drivers: things that make development more difficult software cost drivers: things that affect development time product: complexity measures personnel: capability measures platform: time and storage constraints project: quality of software tools Two kinds of estimation: COSYSMO: model system engineering process. Concerned with how long it takes to design a solution to rough requirements. COCOMO: model software implementation process. Concerned with how long it takes to write the solution.

16 Cost Page 16 Uncertainty Monday, October 05, :11 PM All cost modeling has some uncertainty is your measurement of project complexity accurate? is your description of staff capabilities accurate? are the documented cases used to calibrate the model similar to yours?

17 Cost Page 17 Coping with uncertainty Monday, October 05, :13 PM Coping with uncertainty Estimate pessimistic, optimistic, and likely scenerios. pessimistic: complexity is higher than expected, or equivalently, capability is lower optimistic: complexity is lower than expected, or equivalently, capability is higher. Use pessimistic and optimistic cases to bound cost. Use some average, e.g., (P + 4L + O)/6 as a realistic estimate, where P=pessimistic alternative L=likely alternative O=optimistic alternative