choice-metrics.com

Hi there,

Apologies if this is a stupid question, this is my first project using Ngene.

Essentially, I've generated the design I want and saved it as a NGD file. However, every time I reopen it the blocking has changed - i.e. Choice Situation 1 was in Block 1 but is now in Block 2 etc...

I'm just double checking that this is normal, and not something to do with the way I've saved the file?

I note in the manual it states:

It is worth noting that .ngd files do not store all possible properties of the design. Instead, the file
only contains the syntax used to generate the file, and the design levels. When the design is
opened, the syntax is parsed and used to evaluate the stored design, making all properties
available. This has several advantages, including a small file size and the ability for future
versions of Ngene to report additional properties. The key disadvantage is that opening the .ngd
file may be a little slow for some complex designs.


This implies that the blocking should change every time the NGD file is opened. I was a bit surprised at this, as I thought changing the blocking in this way would change the statistical properties of the design. However, it seems I've been mistaken...

Any information regarding this matter would be great.

Best wishes,

Tom

Dear Tom,

First, note that blocking has no impact on the statistical properties of a design, e.g., it does not effect efficiency or orthogonality.
I generated a design, saved it as NGD, and opened it again and it retained the same blocking, so it seems to store it?

Perhaps you are referring to a design optimised for panel mixed logit (rppanel), where blocking is conducted AFTER the design has been generated? In all other cases, blocking is performed while generating the design.
I have asked a colleague at ChoiceMetrics if they can respond as well, as I am not sure how blocking is saved.

Michiel

Hi Michiel,

Thank you for confirming that the blocking has no impact on the efficiency, orthogonality etc - thinking about this a bit more this makes sense.

It's odd that the NGD is retaining the blocking for you and not for me. I'm not using a design optimised for a panel mixed logit, and the main part of my code is:

;alts = ChoiceA*, ChoiceB*
;rows = 120
;block = 10
;eff = (mnl,d,mean)
;bdraws = sobol(2000)
;model:
[Bayesian priors]

It caused a bit of confusion with the survey company we are dealing with for our survey! If it continues to be an issue we can just resend them the design in an excel file rather than a NGD.

Thank you as ever for your swift reply.

Best wishes,

Tom

You can open the NGD file in a plain text editor (e.g. Notepad in Windows) and there you should be able to see the design as well as the blocking column, it should definitely be stored (I can see the blocking column in my NGD file).

I am sorry to hear that it caused confusion, I am not able to explain why, but perhaps there is a clue in your NGD file? If you would like us to investigate, please send the file to contact@choice-metrics.com.

Michiel

Hi Tom

Can you please check that you are using that latest version of Ngene, 1.2.1? You can check via Help menu/About. Versions of the software prior to 1.1.2 will regenerate the blocking column when a design is opened. Since this is generally undesirable, we changed this in versino 1.1.2.

The ;block entry in the Syntax Reference section of the manual for version 1.2.1 discusses this:
By default, if the design contains a blocking column, the levels of that column
will be preserved if an existing design is opened or evaluated with the ;eval
property. In some instances, the user may wish to generate a new blocking
column. For example, the blocking could be performed for a longer time
period, to improve the quality of the blocking, or the number of blocks could
be altered (in both instances, through changes to the ;block property). To
achieve this, add the 'newblocking' property value.

If you are using Ngene 1.2.1, please email me the .ngd file at contact@choice-metrics.com, and I will investigate.

Andrew

Hi Michiel and Andrew,

Thank you both for getting back to me.

I checked which version of Ngene I was using, and it was version 1.1.1, so this is what will be causing the problem!

I get in touch with my departmental IT service and see if I can get upgraded to the most recent version of the software.

Thank you again for your help with this.

Best wishes,

Tom

Good to hear.

Andrew