As a consequence of the casual relationship evident between presence and the perceived realism of VR, much research has also undertaken the task of determining possibilities for reliable measurement of the concept. While many questionnaires and surveys are available to attempt to measure presence via underlying causal factors and determining variables such as those identified in earlier sections of the chapter, only a small number of these have gained widespread use.

Presence measurements must be reliable. They can achieve this through designs which are dependent only on the considered characteristics, and ensuring validity through measuring only what they are intended to measure [18]. Approaching the concept from different perspectives researchers have as a consequence developed different methods of measuring presence. These attempts are discussed here under the two major types: subjective self-reports, and objective measures.

1. Subjective Measurements of Presence

As described earlier, under considerations of user characteristics, even if users have the same experience in a VE it is unlikely they would report the identical experience. It is for this reason that Slater [57] claims self-report is not appropriate for measuring presence. Subjective self-reports are by nature inexplicably tied to personal aspects of the user. For example, Nisbett and Wilson [58] argue that introspective reports do not function as memories of mental process, but rather, that they are a process of the subject constructing an explanation of their behaviour based on personal theories of behaviour.

Subjective measurements of presence, however, are essential in order to collect the user’s personal opinion [2, 3]. In addition, the majority of methods developed to measure presence to date have relied on subjective measurements using self-report [26, 59]. Subjective measures of presence include distinct forms of evaluation: scales (e.g., from 1 to 10, what level of being there did this virtual environment offer?); paired comparative method (e.g., which system offered more presence?); and comparative method by similarities among distinct modes (e.g., put this light as bright as the strength of presence you have experimented in this VR system).

In utilizing any self-report measure of presence, however, it must be borne in mind that results can be tied to the personal aspects of the user.

1.1 Scales
Subjective evaluation scales have been used extensively to assess presence in virtual environments [63]. As data collection during the exposure could influence negatively the experience of presence it is recommended it be done immediately after the exposure [2].

Commencing with a theoretical body of work, essentially based on Sheridan [3], Held and Durlach’s [45] work, and on a number of empirical studies, Witmer, Jerome and Singer [60] developed and validated the Presence Questionnaire (PQ), including 32 items measured through a 7 point Likert scale, that measures presence after using a VR system via causal factors. The questionnaire has gained a significant level of acceptance and has been tested across a number of studies [61; 62]. The PQ has four sub-scales: a) involvement; b) sensory fidelity; c) adaptation/immersion and d) interface quality. All four sub-scales measure user’s perception of display system features.

Another well-known scale is the ITC-Sense of Presence Inventory (ITC-SOPI). It is a state questionnaire measure that focuses on users' experiences of media, with no reference to objective system parameters [64]. It has been translated in many languages and used in studies covering a wide range of media.
The ITC-SOPI, including 44 items measured through a 5 point Likert scale, has four factors:

- Sense of Physical Space, 19 items: a sense of being located in a physical space depicted by the media system
- Engagement, 13 items: a sense of involvement with the narrative/content of the mediated environment
- Ecological Validity, 5 items: a sense of naturalness and believability of the depiction of the environment itself and events within the environment;
- Negative Effects, 6 items: the negative experiences associated to an immersive media, such as eye-strain, headache, sickness.

Other widely used questionnaires are (for the full list of the available questionnaires see the Presence Research web site: http://www.presence-research.org): the UCL Presence [65] questionnaire (3 items), the Reality Judgement Presence [66] questionnaire (18 items) and the Igroup Presence [67] questionnaire (14 items).

However, it must be considered that by measuring presence using subjective self-report, a conflict is created between a user’s feelings or emotions and their knowledge. For example, the user knows he is in a virtual world and remembers how he entered this new situation yet the investigator is asking him to respond to questions relating to the extent to which he feels present in this artificially entered situation [59].

1.2 Comparative
Presence has been divided into subjective and objective aspects [68]. The first is the likelihood of the person perceiving himself as being physically present in the virtual environment; the second, the feasibility of a task being completed successfully. We would suggest that the subjects be asked to compare the virtual environment with reality in order to measure presence. Because it is still very unlikely that someone would mistake the real world for the virtual presentation, Schloerb [68] suggests a degradation of the real scene through the use of filters, in order to confuse the real and virtual environments. However, this measure might become an assessment of the discrimination ability between two images, instead of the evaluation of presence [1], and “in similarity limit, the answers between the systems would be equivalent” [59].

With current technology it is hard for the participant to confuse the two worlds, and the level of degradation needed can be used as a measure of presence. It is natural that the subject is more sensitive to the degradation of a particular stimulus (e.g., frame rate of image presentation) than another (e.g., sound). It will also be difficult to deteriorate aspects of the real scene in order to fit the virtual scene. One of the advantages of this method would be the lack of need to question the participants about presence directly.

A variant of this method is the “Break in Presence” approach [69]. This approach is based on the idea that a participant experiencing virtual reality technology interprets the stimuli coming from the environment as belonging either to the virtual or to the real world. Slater & Steed suggested that the participant switches between the two interpretations throughout the experience, and that a measure of presence could be obtained if the amount of time that the participant spent interpreting the stimuli as coming from the virtual could be estimated. They proposed to do this estimation by looking for “breaks” those times when the participant realised they were in the real world.

The main limitation of this approach is its oversimplification: it does not address the full complexity of mediated experience. For example, it does not account for mixed perceptions where the participant simultaneously holds and even partially responds to both (real and virtual) interpretations, as noted by Spagnolli and Gamberini [70, 71].

1.3 Similarities between Distinct Modes
In order to assess presence, one can also ask the participant to compare magnitudes in different modes. Pressure and luminance are sometimes used as an example. In this case, the participant presses a button with the strength he believes is correspondent to the level of brilliance of the light. Another possibility is sound and presence: the subject elevates the amplitude of a sound to the level of presence he felt in the VR environment. Although this method has many methodological difficulties it is considered to be an adequate quantitative measure of presence [72].

In order to continuously evaluate presence, Ijsselsteijn and collaborators [1] used an instrument to continuously evaluate image quality in television screens. It consisted of a small sliding part (hand-held slider), that the subject moved forward or backwards, according to the degree of presence they felt. A possible criticism of this measure is that the participant will be dividing his attention between the virtual task and the measure of presence. These authors defend their method by stating that the participants are aware of being in a laboratory, making it unlikely that they would believe they were in fact in the scene presented on screen. Instead, according to the authors, they refer to a feeling of being in the environment similar to the one they would feel if they actually were there.

2. Objective Presence Measurements

Objective performance measures have the advantage of not interfering with the task. However, presence has no physical manifestation objectively measurable [2]. The objective methods to measure presence have mostly used neurophysiologic measures, performance evaluation and postural response evaluation. Each of these forms of evaluation will now be briefly considered.

2.1 Physiological Measures
The use of a high number of neurophysiologic responses like cardiac frequency, skin’s electric conductance (GSR), reflex motor behaviours and VR event evoked cortical responses were suggested as objective measures corroborative of presence. However, available research is still scarce [1]. Authors consider that physiologic reactions should be similar to those observed in a real environment. In a study by Dillon, Keogh, Freeman and Davidoff [38], it was observed that cardiac frequency was higher during the presentation of rally sequences as compared with calm boat sequences.

However, a critical issue for using neurophysiologic responses for presence measurement is the understanding of the link between emotion and presence. In particular its critical to identify what is an emotional response and what is a presence response. As noted by Baños and colleagues [55] within the activity of the “EMMA” European funded research project, there are significant differences between emotional and neutral environments in presence measurements. On one side, the emotional environment seems to be more engaging, natural, believable and real to users than the neutral environment. On the other side, the influence of immersion on presence was higher in non-emotional environments than in emotional ones.

Recently “PRESENCIA” (http://www.cs.ucl.ac.uk/research/vr/Projects/Presencia/), another European funded research project, has tried to identify a neural and physiological characterization of presence. Specifically the project is carrying out different psycho-physiological and brain imaging studies to identify the physiological and neuronal signatures associated with switches between different presence states. The key goal is to implement fMRI experiments, using event-related designs, where the presence state (or switches in state of presence) is indexed by (i) phenomenological report from subjects (ii) a change in bodily state indexed by independent psycho-physiological markers.

2.2 Performance Measures
It is frequently suggested that increased presence will produce better task performance and better skill transfer to the real world [72]. In order for these constructs to have validity, they should first allow the empirical establishment of equivalent classes [73]. This demonstration asks for a registration of performance variation when the factors which influence the construct vary. This also requires that a variation which does not change the construct also does not influence the performance the construct it is supposed to explain. Thus, equivalent classes are of use for the evaluation of change in presence. However, the relationship between presence and performance is unclear, given that performance can actually improve with a decrease in presence [73].

2.3 Postural Responses
According to Steuer [4], immersion is a function of system and user. However it seems likely that one can be physically immersed in a VR system but not ‘be there’, that is, feel presence. In other words, participation in a VR system does not guarantee presence. Immersion is created for a user when they give their attention and commitment to their closest physical environment [4]. Thus, it is the level of commitment to the environment which determines the level of immersion.

According to Usoh, Alberto and Slater [17], an increase in presence raises the similarity between behaviour in virtual environment and usual behaviour in a real environment. The measure of these differences should therefore provide a means of measuring presence. The adjustment of the observer’s posture as a possible corroborative measure of presence has also been explored. It is a distinct and promising measure of presence assessment that consists of postural response evaluation to stimuli presented. Referred to as behavioural realism, the basic principle of this form of assessment is that the more similar the virtual environment is to the one it mimics, the more similar the observer’s response to the virtual presentation will be [59].

These measures are potentially useful for two reasons. First, the observers are not normally conscious of their postural responses, and so their responses are less likely to be affected by subjective assessments [30]. Second, because postural measures have the capacity of producing different levels of responses, they do not simply generate binary results such as yes and no. Instead, it is possible to assess degrees of response and relate them to different degrees of presence [59]. The implicit theory in this form of assessment is that postural changes only occur if the subject is extremely present in the virtual environment. The advantage of this approach is in the evaluation of observable phenomena. Its weakness is lies in a lack of sensitivity to and exclusion of subtle aspects of presence [74].