# Toward a Trust Evaluation Mechanism in the Social Internet of Things
## 3 Trust Evaluation Model: Background and Provisions
### Brief Understanding on How to Evaluate Trust  
As implied in the conceptual model in Section 2.4, a trivial trust evaluation scheme could be as the following procedure: (i) determine and calculate all TAs of a trustee's trustworthiness; (ii) specify task requirements and preferences, (iii) figure out all environment conditions; then (iv) incorporate these factors to build trust. This trust evaluation model is called _direct trust_ that indeed calculates trust based on direct observations on both the participants (the trustor and the trustee) and the environment. However, this approach finds unfeasible to efficiently measure trust due to several reasons. For example, there are variety of TAs (some of them are listed in Table 1) need to be quantified in order to measure the _direct trust_; and this is an impossible mission. One reason for this is due to the ambiguity and variability of natural language when defining terms for TAs that are still debatable in trust literature. Another reason is the complication and limitation of data collection, technologies and methodologies for valuating all the TAs as well as the complexity of incorporating TAs with trustor's propensity and environment conditions to evaluate trust. Authors in [20] also mentioned that TA collection might cause privacy leakage which makes involved entities reluctant to provide personal evidence for a trust evaluation platform.  
Consequently, instead of measuring trust using only the direct trust approach, a prospective approach is to determine a set of indicators called Trust Indicators (TIs) that are feasible, not so complicated to obtain, and cover different aspects of trust. As the word \"indicator\" implies, each TI is as a \"piece of a puzzle\" showing the consensus of trust. TIs could be a TA or a combination of several TAs; could also be a combination some TAs with trustor's propensity and environmental factors. TIs can be obtained using different approaches, for instance, the direct trust evaluation model could produce a good TI. However, other TIs do not necessarily only stick to the direct trust evaluation scheme. Thanks to the integration of social networks, some TIs can be determined based on social interactions in the SIoT environment that effectively indicate trust such as Recommendation and Reputation which are evaluated contingent on the propagation characteristic of trust. These TIs are then combined to derive a portion of the _complete trust_ called _computational trust_. The _computational trust_ is persuasively used on behalf of the _complete trust_ (Figure 3). As many TIs are specified and evaluated as more accurate the _computational trust_ will get. However, as two sides of a coin, there is always trade-off between computational trust accuracy and computational efforts.  
Figure 3: Concept of computational trust that comprised of multiple trust metrics.  
Nevertheless, any trust evaluation models in SIoT environment should determine two objectives: (i) specify a set of TIs in which each TI represents a piece of the three factors: trustee's trustworthiness, the trustor's propensity, and the environmental factor; (ii) propose mechanisms to evaluate the TIs as well as to derive the computational trust value from the TIs. Again, the _computational trust_ should be much similar to the _complete trust_ so that it can be efficiently used on behalf of the _complete trust_ in most of the cases.