I published a conference abstract with a undergraduate student of mine as a co-author. The work was presented as a poster and the conference doesn't publish proceedings/papers. I am now planning on publishing an extended version and want to drop her as a co-author. Can I do this?
Prior to the student starting the project we discussed authorship and agreed that it was unlikely that she would contribute enough to warrant authorship on anything that came out of the study since the experiment was conceived and implemented by me and I designed the statistical analysis. At the time of the conference, the student had collected approximately 1/4 of the data on the poster and did a fair amount of the statistical analysis, but her contribution was essentially just turning the crank. Given the effort she put in, I did not think it was out of place to have her as a co-author on the poster, but nor did I think it was required. I wrote the abstract and poster and asked her if she wanted to be an author and if so for any input on the abstract and poster.
Since then, I have collected a second independent data set and come up with a completely new analysis. I now feel her contribution (1/8 of the total data set and helping with a partial preliminary analysis) does not warrant authorship and I think it would be better to acknowledge her work in the acknowledgements then to add her as an author. I would discuss this with her, but she is out of the field and I cannot contact her easily. Can I drop her for the list of authors?