Specifications

[adikarina]

Hi all,
I am struggling to find some information regarding the different types of specifications and when which type should be used.

Please provide some guidance as where to find it.

Thanks
Aditi

Not surprised; I can't immediately put my finger on one good source. Sure there are several references but I'll need some time to get a selection together; also I think they'll be a need to derive what you need from a composite of sources as background reading rather than a ready written comparison.

In the meantime to get you thinking on the right lines, I suggest you consider what you would do in your own life re a variety of possible purchases. For each of the following categories, how would you decide between a range of items on sale when looking to purchase :
a) a house
b) a car
c) a pair of shoes
d) ingredients for cooking a meal
e) a book to read on a long flight
f) a torch battery
g) a replacement cartridge for your inkjet printer

How would your criteria for making a selection differ in each case?
Which items would you feel appropriate to make "sight unseen" over the internet rather than experiencing the item in person?
Suppose you were a very busy and affluent person and you had a "personal shopper" either actually to make a purchase for you or at least make a shortlist of items for you to choose between; how would you instruct them in your selection criteria?
If you start to appreciate what the differences in your approach may be in the various cases, you'll get some feeling of why different types of specification are appropriate in different scenarios.

[Peter]

Hi all,
I am struggling to find some information regarding the different types of specifications and when which type should be used.

Please provide some guidance as where to find it.

Thanks
Aditi

I am not sure I quite understand what you are asking. Are you talking about formal methods for specifications or something else?

[PJW]

Hi all,
I am struggling to find some information regarding the different types of specifications and when which type should be used.

Please provide some guidance as where to find it.

Thanks
Aditi

Not surprised; I can't immediately put my finger on one good source. Sure there are several references but I'll need some time to get a selection together; also I think they'll be a need to derive what you need from a composite of sources as background reading rather than a ready written comparison.

For a one-pager, which similarly is ostensibly very little to do with railway signalling, look at this

Slightly more detailed and wider ranging is this one

Perhaps the best I have found so far ; it could almost be an answer to an IRSE question (it is of about the right length and structure); it may not be particular industry specific but had it had some more relevant examples...

Another brief example that I have copied and now paste here:
Two types of specifications are commonly used in contracts dealing with landscape and revegetation projects: Performance and Prescription.
A performance spec merely states what the final outcome of the project should be, e.g., ...perennial turfgrass cover, three deciduous trees per one thousand square yards, drainage that meets all current regulations and industry standards....
Such a specification leaves the method used by the contractor up to the contractor. Successful completion of work will be judged on the basis of the project's outcome.

Prescriptive specifications, on the other hand, tell the contractor exactly what to do and how to do it. He will have fulfilled his contract if he plants three prescribed trees at prescribed spots in holes of a prescribed depth; a 100 pounds of seed X and 50 pounds of fertilizer Y using equipment Z.
Nothing has to actually grow in order to meet the prescription.

Does this begin to get you started- can you relate this to your private life / railway experience?
Do you have a particualr IRSE exam question in mind or does this relate to real work in the day job?

[adikarina]

Hi Peter,
Here is a question I was loking at in the module 7 2008 paper:-

Question 7
Provide an outline functional performance specification for procuring train detection equipment for use as the basis of a block signalling system.
[15 marks]
What are the main advantages and disadvantages of providing a functional performance specification rather than a detailed design specification?
[5 marks]

I will look into the information links you have provided and maybe attempt it.

Thanks
Aditi

[PJW]

Hi Peter,
Here is a question I was loking at in the module 7 2008 paper:-

Question 7
Provide an outline functional performance specification for procuring train detection equipment for use as the basis of a block signalling system.
[15 marks]
What are the main advantages and disadvantages of providing a functional performance specification rather than a detailed design specification?
[5 marks]

I will look into the information links you have provided and maybe attempt it.

Thanks
Aditi

I think that looking at those links etc should give you plenty for those last 5 marks.

The first 15 will need you to use your knowledge of train detection criteria but also get you to think about "all the categories of things" that are important for a particular railway and its application. I don't want to lead you too much because I want you to think, but perhaps a few hints are appropriate to get you started as you still seem hesitant.

Earlier I suggested you try to relate to decision to purchase various items in your personal life. You need to think what you really NEED and what you'd LIKE and for this you need to know what you are going to USE the object for and in what ENVIRONMENT.

Writing a specification is basically telling the supplier what is important to you- this may be entirely different to what is important to me. It may also be entirely different from what you wanted last time, because even if it is in some ways similar you may need it for a different role.

So to take one of those examples- buying footwear. I guess you take a different size to me, but each of us are likely to take the same size on each occasion we go into the shop. However on one day you may wish to buy some high heels, on another occasion some training shoes and on another a pair of safety boots. I guess the colour may be an important factor in your choice for the first two, but probably of very little relevance for the latter. Perhaps you wish to avoid wearing any leather objects; in which case that would be an important criteria to you in every case. The anti-slip nature of the sole may be a relevant consideration every time, but more important in the case of the safety boots than the others. Similarly you may try to assess for likely durability, but again that would be relevant more for some purchases than others. So you probably go into the shop with some form of idea of what you are looking to purchase and soome perhaps subjective criteria to judge potential candidate items against.

Writing a specification is just a more formal means of doing this and having some more objective criteria that a "third party" could assess against (so that they could say "yes the object complies" or "no the object does not comply in these respects to this extent"). It isn't as easy as you might think and this is where clients and contractors often get into disagreements. You ask me to supply you with a pair of brown boots (envisaging wearing them at an office party that you think that I know you are attending); I supply a perfectly good pair of brown walking boots (because you did mention that you were soon going on holiday to the Lake District). End result is that you are far from happy and criticise me as incompetent; I think you are acting totally unreasonably and demand payment for what I provided in good faith and is in accordance with what was actually specified as your requirement.

So have a goand see what you come up with. You won't come close to giving a water-tight specification in 20 minutes; however remember that you want as WIDE a consideration as possible- better to have some examples spread over the whole range of possible considerations than to concentrate in too much detail on a few very specific items, thoug a hint that you can do detail in perhaps one key area is also important. Think about the analogy above; over to you.

[Peter]

I gave a short paper at the IRSE YM section AGM which discussed the pros and cons of the approach of being generic vs going down to the nth degree of detail - the very dilemma that PJW is talking about. I used the cartoon on the second slide here to give a humorous look at how what one person says can be taken to mean something different - never assume anything.

[PJW]

I gave a short paper at the IRSE YM section AGM which discussed the pros and cons of the approach of being generic vs going down to the nth degree of detail - the very dilemma that PJW is talking about. I used the cartoon on the second slide here to give a humorous look at how what one person says can be taken to mean something different - never assume anything.

I am getting old; I knew that I had recently seen something that would have been useful for Aditi but I couldn't recall what. It wasn't a written source but your presentation. Couldn't you post it here for the benefit of those who didn't attend in January?

[Peter]

I am getting old; I knew that I had recently seen something that would have been useful for Aditi but I couldn't recall what. It wasn't a written source but your presentation. Couldn't you post it here for the benefit of those who didn't attend in January?

I'll get a copy of the other computer and try to write up some of the words that I recall using as the slides don't tell the whole story on their own.

[Peter]

I am getting old; I knew that I had recently seen something that would have been useful for Aditi but I couldn't recall what. It wasn't a written source but your presentation. Couldn't you post it here for the benefit of those who didn't attend in January?

I'll get a copy of the other computer and try to write up some of the words that I recall using as the slides don't tell the whole story on their own.

Attached are two files - one has the presentation slides and the other the rough notes for the words that I was trying to follow. It is not intended as an answer about how a specification should be written, more an illustration of the problems you can encounter.

[PJW]

What are the main advantages and disadvantages of providing a functional performance specification rather than a detailed design specification? [5 marks]

To compare specifications of different types, see attachments in this mod 5 thread