Setting

In this study, four neighbourhoods that represented a variety in residential area density (RAD) and socio-economic status (SES) were selected for assessing the validity and reliability of the SPOTLIGHT-VAT. These four neighbourhoods were part of the 12 Dutch urban neighbourhoods located in the 'Randstad', selected for assessment of obesogenic environments in the broader SPOTLIGHT project [35, 36]. The urban agglomeration ‘Randstad’ encompasses the four largest Dutch cities and their surroundings in the West of the Netherlands. “Neighbourhoods” were defined according to administrative boundaries as made by the CBS (Statistics Netherlands, http://www.cbs.nl). Data on RAD were obtained from the Urban Atlas database [35]. This atlas is a GIS database distributed by the European Environmental Agency, based on a compilation of satellite photographs covering Europe and providing high resolution land use data [37]. Two classes of RAD were used: high and low residential area density (>80% and < 50% of areas covered by residential buildings, respectively). Socio-economic status data were based on median income data for each neighbourhood from the Netherlands census database [36]. Two classes of SES were used: high and low SES (corresponding to the first and third tertiles). The combination of the RAD and SES levels allowed the definition of four categories (High RAD/High SES, High RAD/Low SES, Low RAD/High SES and Low RAD/Low SES). For the broader SPOTLIGHT project, three neighbourhoods in each category were randomly chosen. In this validation study, we selected one neighbourhood from each category based on feasibility and proximity to the study site. Furthermore, we made sure that the raters did not conduct audits in the city they lived in.

Development of the S-VAT

The S-VAT contains 40 different items and was based on previously published and validated tools [12, 22, 24, 29–34]. The tool was designed for the assessment of dietary and physical-activity-related environmental characteristics within neighbourhoods. The S-VAT was pilot-tested in 32 street segments, and refined accordingly. The final S-VAT incorporated items in eight main categories: walking related items (6 items), cycling related items (8 items), public transport (2 items), aesthetics (9 items), land use-mix (3 items), grocery stores (5 items), food outlets (6 items) and recreational facilities (3 items). The items 'type of street' and 'condition of sidewalk' are included twice in different categories. Each of the categories included multiple items, as depicted in Table 1. For a more detailed description of the individual items we refer to Additional files 1 and 2. A specific S-VAT data input form was created using Open Office open source software, with drop-down menu options for all responses (Figure 1). The data input form allows storage of images (screen capture) from GE. The form was designed to be viewed alongside the Street View feature in GE using a computer split-screen. To increase homogeneity between audits and to assist in clear and unambiguous scoring, a Standard Operating Procedure (SOP) was developed. The SOP describes information on the definition of street segments as well as the procedures for data extraction, data storage and defining environmental characteristics (Additional file 1).

Category	Inter observer reliability		Intra observer reliability		Criterion validity
	% Agreement	Kappa	% Agreement	Kappa	% Agreement	Kappa
Presence of walking related items:*	92.6	0.740	95.1	0.804	97.0	0.856
Type of street (4 categories)	99.2	0.980	100	1.000	97.7	0.938
Presence of sidewalks (yes/no)	89.8	0.764	81.3	0.589	99.2	0.983
Condition of sidewalk (3 categories)	85.9	0.724	95.3	0.907	91.4	0.830
Pedestrian crossing available (yes/no)	95.3	0.642	96.9	0.761	96.9	0.761
Type of pedestrian crossing (if available)	94.5	0.589	96.9	0.765	96.9	0.766
Presence of streetlights (yes/no)	90.6	N/A**	100	N/A	100	N/A
Presence of cycling related items:*	87.1	0.628	95.3	0.849	94.4	0.823
Type of street (4 categories)	99.2	0.980	100	1.000	97.7	0.938
Presence of bicycle lanes (yes/no)	99.2	1.000	99.2	0.973	98.4	0.947
The speed limit in this segment	59.4	0.442	98.4	0.975	96.9	0.950
Obstacles present on bicycle lanes (yes/no)	89.1	0.085	93.8	0.531	95.3	0.557
Do cars form an obstacle on cycle lane (yes/no)	82.8	0.523	82.8	0.588	78.9	0.466
Traffic calming devices present (yes/no)	68.8	0.394	89.8	0.775	89.8	0.777
Public bicycle renting facilities (yes/no)	99.2	N/A	100	1.000	100	1.000
Type of bicycle lanes	99.2	0.973	98.4	0.948	98.4	0.949
Presence of public transport:*	98.9	0.906	99.2	0.946	98.9	0.923
Presence of bus/tram stop (yes/no)	97.7	0.811	98.4	0.892	97.7	0.845
Presence of railway/underground station (yes/no)	100	1.000	100	1.000	100	1.000
Aesthetics:*	78.6	0.440	91.7	0.654	87.3	0.539
Green/water area visible (yes/no)	69.5	0.393	90.6	0.793	88.3	0.740
Residential gardens visible (yes/no)	88.3	0.756	96.1	0.919	96.1	0.919
Rating of most residential buildings (3 categories)	85.2	0.416	93.8	0.796	92.2	0.769
Abandoned or vacant building/area visible (yes/no)	92.2	N/A	96.1	0.741	91.4	0.375
Maintenance of green areas (3 categories)	54.7	0.128	93.8	0.594	85.9	0.347
Condition of sidewalks (3 categories)	85.9	0.724	95.3	0.907	91.4	0.830
Presence of graffiti (yes/no)	90.6	0.306	89.1	0.590	82.8	0.454
Presence of litter (yes/no)	46.1	0.010	76.6	0.520	60.2	0.168
Presence of trees (yes/no)	95.3	0.786	93.8	0.698	97.7	0.883
Land use-mix:*	80.1	0.285	91.4	0.762	91.1	0.740
Presence of residential buildings (yes/no)	85.2	0.416	94.5	0.818	94.5	0.829
Type of residential buildings (5 categories)	72.7	0.000	89.1	0.836	90.6	0.861
% of non-residential buildings (5 categories)	82.5	0.440	90.5	0.633	88.1	0.531
Presence of grocery stores:*	99.2	0.697	100	1.000	99.4	0.681
Supermarket (yes/no)	98.4	N/A	100	1.000	99.2	0.663
Local food shop (yes/no)	97.7	0.393	100	1.000	99.2	0.886
Street food market (yes/no)	100	N/A	100	N/A	100	N/A
Wine/liquor store (yes/no)	100	N/A	100	N/A	100	N/A
Convenience store/small grocery store (yes/no)	100	1.000	100	1.000	98.4	0.494
Presence of food outlets:*	99.2	0.579	99.7	0.887	99.9	0.887
Restaurant (yes/no)	98.4	N/A	99.2	N/A	100	N/A
Fast food restaurant (yes/no)	98.4	0.494	99.2	0.663	99.2	0.663
Take away restaurant (yes/no)	99.2	N/A	100	1.000	100	1.000
On-street vendors of food (yes/no)	100	N/A	100	N/A	100	N/A
Café/bar (yes/no)	99.2	0.663	100	1.000	100	1.000
Shopping mall (yes/no)	100	N/A	100	N/A	100	N/A
Presence of physical activity facilities:*	96.4	0.484	99.0	0.883	97.1	0.527
Indoor recreational facility (yes/no)	100	1.000	100	1.000	99.2	0.663
Outdoor recreational facility (yes/no)	96.9	-0.012	100	1.000	96.9	0.317
Public park (yes/no)	92.2	0.465	96.9	0.650	95.3	0.602
Overall***	91.5	0.595	96.4	0.848	95.6	0.747

*Mean results of Percentage Agreement and Kappa for reliability and validity tests per category.

**N/A: not applicable.

***Mean results of Percentage Agreement and Kappa for reliability and validity tests summarized for all categories.

Procedure

We assessed ten randomly chosen streets in each of the four Dutch neighbourhoods using the S-VAT. Comparability of streets was secured by dividing streets into segments. Street segments were defined as the part of the street between two intersections (with a minimum length of 50 meters and a maximum length of 300 meters) [14]. If streets crossed neighbourhood boundaries, they were assessed entirely or for 300 additional meters (when the street continued for more than 300 meters outside the neighbourhood boundary) [14]. The SOP for the virtual audit (Additional file 1) was adjusted in order to be suitable for the field audit (Additional file 2). Street View images are typically captured from a vehicle. Therefore areas prohibited for cars are generally less visible in Google Street View. Approximately five segments assessed in the current study were pedestrianized and not accessible to the Google car. However, because these segments were short and to a large extent visible from covered sections, we were able to rate these streets according to protocol.

The time taken to complete the field audit and the virtual audit was recorded. The same streets in each neighbourhood were selected to test the validity and reliability. As such, a total of 40 streets (128 street segments) were included in four neighbourhoods. Before the virtual audit took place, the assessors received training on using the S-VAT in the Street View feature of GE. To prevent bias, researchers did not conduct audits in the city in which they lived.

Inter-observer reliability

The first researcher (JRB) performed virtual audits in the four selected neighbourhoods. To test the inter-observer reliability of the virtual audit, a second researcher (JDM) conducted virtual audits in the same streets per neighbourhood, independently from the first auditor. The two researchers were blinded to each other’s results.

Intra-observer reliability

To test the reliability of the S-VAT over repeated audits, the virtual audit was conducted twice by the same auditor (JRB) to measure intra-observer variability. In order to reduce the effect of possible recall bias, the streets were audited in reverse order with at least a ten-day delay between the two audits.

Criterion validity

In order to test the criterion validity of the S-VAT, the virtual audits were compared with field audits for 40 streets in total (ten per neighbourhood). The same data input form and codebook were used as in the virtual audit. The first researcher (JRB) conducted the virtual audit of the same street segments as the field audit, but in reversed order and with an interval of at least ten days between the field and virtual audit. Consequently, the researcher was less likely to be biased by his previous experiences of auditing those street segments.

Statistical analysis

The inter/intra-observer reliability and criterion validity were measured by using Cohen’s Kappa (k). As low Kappa values can be seen despite reported high levels of agreement [14, 38], we also reported the proportions of agreement as described by de Vet et al. [38]. Kappa values of 0.80-1.00 were considered to represent high agreement, 0.60-0.79 as substantial agreement, 0.40-0.59 as moderate agreement, 0.20-0.39 as fair and 0.00-0.19 as slight agreement [39]. As sensitivity analysis, we explored whether validity and reliability across neighbourhood types. However, as shown in the Additional file 3: Table S1, there was not enough variability in items to calculate accurate validity and reliability statistics between neighbourhood types. All analyses were performed using SPSS (version 22).

Intra-observer reliability

There was a high degree of conformity between the first and second virtual audit (96.4% overall agreement, k = 0.848), ranging from 91.7% agreement (k = 0.654) to 100% agreement (k = 1.000). When examining the results in more detail, the lower Kappa scores seen in Aesthetics mostly arose from variables such as 'Litter' (k = 0.520) and 'Graffiti' (k = 0.590).

Inter-observer reliability

Inter-observer reliability results ranged from substantial (78.6% (k = 0.440)) to high agreement (99.2% (k = 0.579)), with an overall agreement of 91.5% (k = 0.595) between the two observers. Similar to the intra-observer reliability results, Aesthetics and Land use-mix were found to have the lowest Kappa scores (k = 0.440 and 0.285) and percentage agreement (78.6% and 80.1%). The items 'Litter' with 46% agreement (k = 0.010) and 'Maintenance of green areas' with 54.7% agreement (k = 0.128) resulted in poor agreement in the category Aesthetics. In the category Land use-mix, the item 'Type of residential buildings' with 72.7% agreement (k = 0.000) is responsible for the fair agreement. In contrast, high percentage agreement (98.9%) and Kappa (k = 0.906) were reported in the category Public transport attractiveness.

Criterion validity

Agreement between the virtual and street audits was substantial to high for most of the categories ranging from 87.3% agreement (k = 0.539) to 99.9% agreement (k = 0.887) with an overall score of 95.6% agreement (k = 0.747). For Aesthetics and Physical activity facilities, agreement results were moderate. Variables such as 'Litter' (60.2% agreement, k = 0.168), 'Maintenance of green areas' (85.9% agreement, k = 0.347) and 'Outdoor recreational facilities' (96.9% agreement, k = 0.317) scored lowest. However, percentage agreement in all categories was found to be high (87.3% - 99.9%).

Strengths and limitations

Some potential limitations with regard to this study and the S-VAT should be noted. First, this study was conducted in Dutch neighbourhoods only. Therefore, generalization of results to environmental studies in other countries should be done with caution, since further inter country validation is needed. The tool however, appears particularly promising for performing inter country comparisons, using standardised methodology at lower costs and less time. Moreover, the use of different types of neighbourhoods (high SES, low SES, high RAD, low RAD) further increased the representativeness of the study.

Second, due to pragmatic reasons, the inter observer reliability and validity tests were conducted with only one other observer. Comparison with additional observers could strengthen the results. Third, there are some specific potential issues related to the temporal validity of GSV images [14, 17]. For instance, on-street vendors of food and obstacles present on the cycling lane are temporally variable and may or may not be present at the time the GSV vehicle drives by. Besides, we noted that GSV updated the imagery between the ratings of the first and second observer; this may have affected the inter rater reliability [22]. Also, the sometimes outdated images could have affected the criterion validity, as the oldest images dated from 2008 and the field audits were conducted in 2014. In the present study it was not feasible to assess differences between images taken over different years. However, Google is currently implementing the option to view images from several years. This may enable the assessment of changes in the physical environment over time. Finally, most GSV imagery was only available in areas accessible to cars. In areas where cars are prohibited, it is generally not possible to conduct virtual audits. Such pedestrianized areas may however, be particularly appealing for physical activity. Nevertheless, when the entrance of a car prohibited street was visible, all visible items were scored. As the street segments that were not accessible to the GSV car were often relatively short, all assessed segments in this study provided suitable data.

One of the strengths of this study is the relatively large number of different items assessed in the SPOTLIGHT-VAT in comparison to other tools [12, 22, 24, 29–33], resulting in a broad-based tool which is the first to assess neighbourhood environmental dietary and physical activity related characteristics. Also, the coverage of street segments in different neighbourhoods adds to the strength of the study, allowing a detailed view of neighbourhoods. What further adds to the strength of this research is that we were able to not only assess the criterion validity, but also the intra-observer reliability and inter-observer reliability. The use of percentage agreement in conjunction with Kappa may be seen as another strength since low Kappa coefficients may be reported despite high levels of agreement [14].