# Practical Approach to Designing and Implementing a Recommendation System for Healthy Challenges
## 2 Materials and Methods
### Challenges Characterization  
In order to make well-informed decisions, it is essential to thoroughly characterize each challenge and assess its relevance in relation to the user's specific dimensions. By conducting a meticulous analysis of pertinent factors, a model comprising 24 features, based on expert consensus, was developed to represent these challenges. These data align with the established user characterization in terms of features and dimensions.  
These values will play a critical role in the challenge selection process as elaborated upon in a two-step approach.  
Initially, we will utilize eight of these features to eliminate challenges that are evidently incompatible, such as not being within the appropriate age group or having incompatible medical conditions. Secondly, up to 16 other features will be employed to assess the suitability of the challenge for the user being evaluated. The former ones correspond to the same items used for the bio-physical and sociodemographic characterization of the users. For each one of them, response options will be provided to properly characterize each challenge in relation to the user's characterization.  
The latter features or dimensions, up to 16, introduced in the model to find out the most suitable challenge are associated with how beneficial the challenge is for each of the dimensions assessed by the SF-36, sHEI-15 and OSC questionnaires. These pieces of data are completed with an estimate of the difficulty of the challenge by means of an item called Effort Required (ER) assessed by the mentioned experts. Table 2 shows a schematic of the items that allow characterizing a challenge and the possible response values for each one.  
\begin{table}
\begin{tabular}{l l} \hline \multicolumn{1}{c}{**Feature/Dimension**} & \multicolumn{1}{c}{**Set of Possible Values**} \\ \hline Age group & 5–17 years. 18–64 years. 65 years or more. \\ BMI \(<\) 18.5 = underweight. 18.5–24.9 = normal weight. 25–29.9 = preobesity. 30–34.9 = class I obesity. 35–39.9 = class II obesity. \(>\)40 = class III obesity (World Health Organization, 2010). \\  & Skin diseases. Pregnancy, childbirth, or puerperimum. Sleep or wakefulness disorders. Endocrine diseases (e.g., diabetes). Diseases of the circulatory system (e.g., hypertension). Diseases of the musculoskeletal system or connective tissue (e.g., arrhythmias, chondopathies). None. \\ Incompatible Injuries & Upper body. Lower body. Trunk. None. \\ Daily time required & \(<\)15\({}^{\prime}\). Between 15\({}^{\prime}\) and 30\({}^{\prime}\). \(>\)30\({}^{\prime}\). Any. \\ Development place & Pool. Sea. Country or city. House. Gym. Any. \\ Required material & Bicycle. Barbells. Sliding discs. Jumping rope. No material. \\ PF & Rating for the physical function dimension (0–10) \\ BP & Rating for the bodily pain dimension (0–10) \\ GH & Rating for the general health dimension (0–10) \\ V & Rating for the vitality dimension (0–10) \\ \hline \end{tabular}
\end{table}
Table 2: Description of the items used for the characterization of each challenge.