## 1 Introduction  
One of the most delicate tasks that science universities are confronted with is the instruction and assessment of their students in using laboratory equipment and successfully carrying out experiments. Serious risks lurk when making use of an on-site laboratory, such as those of accidents and equipment damages, restricting, therefore, the full use of the various instruments as well as the possibility of learning by trial and error. Distance learning universities offering science education face another problem, too: that of not having their students on a regular basis (or at all) at their laboratory facilities, making their lab training and evaluation even harder. The development of virtual laboratories as training platforms complementing on-site lab training can go a long way to address how one becomes familiar with using specialized equipment. In virtual labs, the user would make unlimited virtual use of the various instruments and perform experiments without the fear of equipment damages or accidents. Users could also have the opportunity to have their (virtual lab) performance evaluated with respect to conducting a particular experiment either by a human expert supervising the session or by the virtual lab software itself. This vision (and necessity) has been driving Hellenic Open University's (HOU) development of Onlabs, a 3D virtual biology laboratory for the at-a-distance lab training and evaluation of undergraduate and postgraduate natural science students. The underlying concept is that students can use the software at home before using the actual equipment on-site. Unlike with on-site lab training, students can freely experiment with the lab instruments and equipment in virtually unlimited ways, even making improper use of some of them, and learn from their own mistakes. Additionally, they can make use of an instruction mode, where they are guided with voice and text throughout a particular experiment, or of an evaluation mode, where they obtain a system-calculated score of their performance in an experiment. Onlabs has been developed to resemble a computer game; it contains state-of-the-art 3D graphics, whereas user-environment interaction allows mouse-and-keyboard-based navigation, object handling, knob-turning, switch-clicking, etc. (see Figure 1). Onlabs was initially developed using Hive3D ([[https://www.eyelead.com/index](https://www.eyelead.com/index)]([https://www.eyelead.com/index](https://www.eyelead.com/index)), accessed on 1 July 2024), but development switched to Unity in 2016. There exists ([[http://onlabs.eap.gr/](http://onlabs.eap.gr/)]([http://onlabs.eap.gr/](http://onlabs.eap.gr/)), accessed on 1 July 2024) a collection of versions, either as stable systems or research prototypes, but for the scope of this work, we will focus on one version (2.1.2), which contains two distinct simulated experimental procedures, microscoping and 10X TBE solution preparation, and three different modes of playing, _instruction_, _evaluation_ and _experimentation_. In the _microscoping of a test specimen_, the user is expected to set up the microscope, create a test specimen, and microscope it with all objective lenses of the photonic microscope; in the _preparation of 500 mL of 10X TBE water solution_, the user measures 17.4 gr of Boric Acid and 54 gr of Trizma Base powders on the electronic scale, dissolves them in water with a magnetic stirrer, and adds extra water as well as EDTA pH 8.0 (ethylenediamine tetraacetic acid) to the produced solution. As far as playing modes are concerned, in _instruction_, a user is guided with voice and text through the steps of the selected procedure, each time only being allowed to perform the suggested move; in _experimentation_, a user may make any action whatsoever (provided it has been implemented); and in _evaluation_, a user can, additionally, receive a numerical score reflecting the extent to which the simulated experiment has been completed successfully. It is this latter step (evaluation) that is the focal point of this work.