Banco Santander · Data Scientist
Updated · 2026-09-22

Banco Santander Data Scientist
Interview Questions & Guide 2026

THE 60-SECOND BRIEF

As a Data Scientist at Banco Santander, you will be stepping into a pivotal role at one of the world's largest and most influential financial institutions. Data is the lifeblood of modern banking, and your work will directly impact how the bank manages risk, prevents fraud, and personalizes the financial experience for millions of global customers. You will not just be analyzing data; you will be building the intelligent engines that drive strategic business decisions across multiple retail and corporate banking divisions.

Learn the economics of the product category before the loop. Marketplaces, subscription products and ad-supported products turn on different core quantities (match rate and liquidity, retention and churn, fill rate and yield) and fail in different characteristic ways.

Banco Santander candidates report 2 rounds · ≈ 2-4 weeks. The stages below are what candidates describe, not a published process.

Separate authorization, settlement and dispute outcomes cleanlyReport only matured cohorts for loss metricsRead vintage curves, not blended portfolio averages

27 min read

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See the practice prompts

14Practice promptsAcross five skill areas
3With worked solutionsIncluded in the practice prompts

As a Data Scientist at Banco Santander, you will be stepping into a pivotal role at one of the world's largest and most influential financial institutions. Data is the lifeblood of modern banking, and your work will directly impact how the bank manages risk, prevents fraud, and personalizes the financial experience for millions of global customers. You will not just be analyzing data; you will be building the intelligent engines that drive strategic business decisions across multiple retail and corporate banking divisions.

The impact of this position is immense, touching everything from real-time transaction monitoring systems to advanced credit scoring models. By leveraging massive, complex datasets, you will help the bank optimize its product offerings, streamline operations, and protect vulnerable users from financial crime. Your models will be deployed at an enterprise scale, meaning the solutions you engineer must be robust, compliant, and highly performant.

Expect a role that balances deep technical rigor with high-level strategic influence. You will tackle ambiguous problem spaces, requiring you to translate vague business challenges into concrete machine learning solutions. Whether you are embedded in the risk modeling team or the customer analytics hub, you will find a dynamic environment where your technical expertise directly translates into measurable global impact.

01

Initial Interview

reported

Rounds outside the standard loop often open with something deliberately under-specified: a loose business problem, an open question about a product area, a dataset described in one sentence. The common failure is surveying, listing six plausible approaches and committing to none of them. The thing that separates a strong answer is scoping out loud. State what you are treating as the goal, name the metric you would move, say what you are choosing not to do and why, then take one path through to an actual answer. An interviewer can follow you down a narrow path. Nobody can grade a menu.

What to demonstrate

  • Whether you turn an ambiguous prompt into a stated question with a measurable outcome before doing any work
  • The judgement visible in what you cut, and whether you say why you cut it rather than silently dropping it
  • Whether you land on a concrete recommendation with its caveat attached, rather than an unranked set of options

How to prepare

  • Take three vague prompts, such as 'is this feature working', 'why did retention drop', and 'should we expand into a new segment'. For each, write one sentence of goal, one primary metric with its window, and two things you are explicitly not doing.
  • Practise giving the recommendation first and the reasoning second, in five minutes. Loosely defined rounds are usually time-boxed, and an answer that arrives last often does not arrive.
  • Keep a running assumption list as you talk, on paper or in the shared doc, so the interviewer can challenge one assumption instead of your whole answer.
PracHub interview research ↗
02

Technical Deep Dive

reported

Before anything else, this round is a reading test. You are given a small schema and a question phrased in business language, and most of the difficulty sits in the gap between them. Who counts as an active user, does a refunded order still count as an order, is that date column an event time or a load time. Weak answers start typing immediately and compute something precise about the wrong population. Strong ones pin the definition in one sentence, name the column that encodes it, then write the query. On a timed assessment with nobody to tell, write the definition in a comment anyway.

What to demonstrate

  • Whether an ambiguous term becomes a specific column and filter before any computation happens
  • Whether you read the schema for keys and cardinality rather than only for column names
  • Whether the result answers the question at the grain it was asked at, per user or per session or per day

How to prepare

  • Take three metrics you already use and write down the exact filter and exact grain behind each, then practise stating one of them in a single sentence out loud
  • On a schema you have never seen, spend the first minute writing what one row of each table means and which key it is unique on, then predict which joins can duplicate rows
  • Rehearse a version where the definition changes halfway through, and edit the query you have instead of starting over
PracHub interview research ↗

PracHub editorial advice for the preparation topics above.

01

Using written premium as the denominator of a loss ratio

Premium is written at inception and earned pro rata across the exposure period, so in a growing book written premium runs ahead of earned premium and the loss ratio comes out too low, with the error reversing when the book shrinks. The numerator has the mirror-image problem if it omits incurred-but-not-reported reserves, since recent accident periods then look profitable twice over. Both sides must refer to the same exposure period, which is what an accident-period view at a fixed development age enforces.

02

Reading the most recent months of fraud and dispute rates as final

Consumer dispute rights commonly run around 120 days from the transaction or expected delivery date, and several reason codes run considerably longer, so the disputes belonging to a recent transaction month have simply not been filed yet. Any chart attributed by transaction date therefore slopes down at the right edge regardless of what is happening. The fix is to report only matured cohorts, or to apply development factors estimated from completed months and to show the estimate as an estimate.

03

Generalising beyond the population the sample actually supports

State the frame the sample was drawn from and where it diverges from the population you want to talk about: time window, platform, geography, opt-in. If a group is excluded from the frame, either weight to known margins or narrow the claim rather than quietly extending it.

04

Dropping rows with missing values without naming the mechanism

Say whether the values are missing at random, missing by a known process, or missing in a way that depends on the outcome, and handle them accordingly. Deleting incomplete rows silently redefines the population whenever missingness correlates with what you are measuring.

Choose a category, try a prompt, then open its approach, worked solution or follow-up when you need it.

11 technical prompts3 include a worked solution

How do you handle multicollinearity in a multiple linear regression mo…

medium
machine learning and modelling

How do you handle multicollinearity in a multiple linear regression model?

Approach
  1. Set a baseline first, so any model has something honest to beat.
  2. Pick an evaluation metric that matches the cost of each error type, not a default.
  3. Check what information would not exist at prediction time, and exclude it.
Follow-up
  • How would you choose the decision threshold, and who owns that choice?
  • Where could label leakage enter this setup?

Walk me through how you would build a model to predict customer defaul…

medium
machine learning and modelling

Walk me through how you would build a model to predict customer default on a personal loan.

Approach
  1. Say how the offline result would be validated online before it is trusted.
  2. Frame the prediction: the label, the moment of prediction, and the action it triggers.
  3. Pick an evaluation metric that matches the cost of each error type, not a default.
Follow-up
  • How would you choose the decision threshold, and who owns that choice?
  • Where could label leakage enter this setup?

Simulate false alarms in a merchant chargeback monitoring rule

mediumWorked solution
simulationrare eventsmonitoring thresholds

Baseline matured first-chargeback rate is 12 per 10,000 settled transactions. A monitoring rule alerts when a merchant's observed monthly rate exceeds twice baseline. For monthly settled transaction counts of 500, 2,000, 10,000 and 50,000, simulate the false-alarm probability per merchant-month under the baseline, and the power to detect a merchant whose true rate is 30 per 10,000. Then, for a portfolio of 4,000 merchants split 60, 25, 10 and 5 percent across those four counts, give the expected number of false alarms per month.

Approach
  1. Recognise the rule is a threshold on an integer count, not on a continuous rate. At n = 500, twice baseline is 24 per 10,000, so the first observable value above it is 2 chargebacks, or 40 per 10,000. Derive the trigger count for every n before simulating anything.
  2. Draw binomial counts with numpy at p = 0.0012 and take the share at or above the trigger for the false-alarm rate, then repeat at p = 0.0030 for power. Use at least 200,000 draws per cell so a probability near 0.001 has a usable standard error.
  3. Cross-check every simulated cell against the Poisson approximation with lambda = n*p, which is tight here because p is tiny. A mismatch almost always means the trigger count is off by one.
  4. Weight the per-merchant false-alarm probabilities by the portfolio mix, and report the share of expected alerts contributed by each size band rather than only the total.
  5. Close on the operating consequence: a fixed multiplicative threshold is not a constant false-alarm rate across merchant sizes, so either the threshold scales with n or small merchants need a minimum volume before the rule applies.
Worked solution 30 min
  1. For each n, compute trigger = floor(2 * 0.0012 * n) + 1 and print the four values before simulating.
  2. Simulate 200,000 binomial draws per n at p = 0.0012 and take the share at or above the trigger.
  3. Repeat at p = 0.0030 and record power for the same triggers.
  4. Compute the Poisson tail 1 - CDF(trigger - 1, lambda = n*p) for both p values and confirm agreement within Monte Carlo error.
  5. Multiply the false-alarm probabilities by 2400, 1000, 400 and 200 merchants and sum.
EXPECTED RESULTTrigger counts of 2, 5, 25 and 121. False-alarm probability roughly 12 percent at n = 500, roughly 10 percent at n = 2,000, under 0.1 percent at n = 10,000 and effectively zero at n = 50,000. Power against 30 per 10,000 roughly 44, 72, 85 and above 99 percent. Expected false alarms about 390 per month, with over 99 percent of them coming from the two smallest bands.
Follow-up
  • How would you set a threshold that holds the false-alarm rate roughly constant across merchant size?
  • The rule reads the transaction month, but disputes arrive for up to 120 days afterwards. What does that do to the alert and how would you fix it?
  • What does a month of these false alarms cost, and how would you decide whether it is worth paying?

For someone who can already write the query and train the model but stalls when asked what to measure or whether a change is worth making. Metric definition and case structure come first; the technical work is kept as maintenance rather than the centre of the week.

Small steps. Visible outcomes.0 / 7 completed
ONE WEEK · YOUR PACE

Prepare, practise & reflect

One practical outcome each day. Spend longer where you need it.

0 / 7 done
01Metric anatomy
  • For three products you use daily, write one primary metric, two input metrics that plausibly move it, and one guardrail that would catch a cheap way of moving the primary at the cost of the product.
  • For one of them, specify the metric precisely enough that two analysts would return the same number: numerator, denominator, unit of observation, time window, and how returning and deleted accounts are treated.
  • Pick a ratio metric and write what happens to it when the denominator shrinks for reasons unrelated to the numerator, with a concrete example of that happening.

Deliverable: A one-page metric tree for one product, with the primary metric written as an unambiguous spec.

Practice prompt ↗Practice prompt ↗Worked solution ↗
02Diagnosing a drop without guessing
  • Take the prompt "weekly active users fell 8 percent week over week" and write the segmentation plan before proposing any cause: platform, region, tenure cohort, acquisition channel, and whether the movement sits in the numerator or in a changed denominator.
  • List the instrumentation failures that manufacture fake drops (a client release that stopped firing an event, a bot filter change, a shifted date boundary or timezone) and write the query that rules out each one.
  • Rehearse stating the boring explanations first, seasonality and day-of-week composition, before reaching for a product cause.

Deliverable: A drop-diagnosis checklist short enough to recite from memory in under a minute.

Practice prompt ↗Practice prompt ↗
03Should we build it
  • Take a feature idea and write it as a bet: what you believe is true, what would have to be true for it to pay off, the metric that would confirm it, and the effect size that would justify the engineering cost.
  • Size the opportunity top-down and bottom-up, then reconcile the two numbers in writing instead of quoting whichever is friendlier.
  • Write the counter-metric that would make you kill the feature even if it wins on the primary metric.

Deliverable: A one-page product memo ending in a decision rather than a list of considerations.

Practice prompt ↗Practice prompt ↗
04The places aggregate numbers lie
  • Construct a Simpson's paradox numerically: two segments where the treatment wins within each segment yet loses overall, and identify the shift in segment weights that causes it.
  • Take a heavy right-tailed quantity such as revenue per user and write why the mean is the wrong summary, which percentile you would report instead, and what a moving mean with a stable median tells you.
  • Write your definition of a session for the product from day one, then name two real behaviours it misclassifies.

Deliverable: One page holding a worked Simpson's paradox table and a session definition with its two known failure cases.

Practice prompt ↗Practice prompt ↗Worked solution ↗
05Technical maintenance, aimed at metrics
  • Solve four timed SQL prompts that all end in a ratio metric, so the question of grain stays live in every answer.
  • Compute a 95 percent confidence interval for a proportion on a small sample, and state why the normal approximation is unreliable when either np or n(1 minus p) falls below roughly 10, along with which interval you would use instead.
  • Take one metric from your day-one tree, write the query that computes it correctly, then write the query that computes it wrong in the most plausible way and explain how you would notice.

Deliverable: Four solved prompts plus a matched correct and plausible-wrong query for one metric.

Practice prompt ↗Practice prompt ↗
06Turning engineering work into data science stories
  • Write three project stories as situation, decision, trade-off, outcome, each carrying one number and one thing you got wrong.
  • For the story you will lead with, prepare an answer to "what would you do differently" that names a decision you made, not a constraint you were handed.
  • Practise the sentence that reframes a systems project as a question project: the question the work answered, ahead of the pipeline it shipped.

Deliverable: Three written stories with the lead story delivered aloud and timed under four minutes.

Practice prompt ↗Practice prompt ↗
07Mock case and gap list
  • Run a 40-minute mock case with someone playing a product manager who pushes back on your metric choice, and record it.
  • Listen back and mark every moment you proposed a solution before the success metric existed.
  • Rewrite those moments as the question you should have asked, and rehearse the first 90 seconds of the case until scoping comes before solving.

Deliverable: A recorded case plus a rewritten opening 90 seconds.

Practice prompt ↗Practice prompt ↗Worked solution ↗

Expand any day for tasks and deliverables. Your progress is saved on this device.

Nearly every data role forces a trade between the analysis you want and the one that fits the decision window. Prepare a case where you deliberately shipped something less rigorous, named the weakness to the person relying on it, and said what would change your answer. The naming is the part interviewers listen for.

Describe a time when your data analysis contradicted a stakeholder's g…

medium
behavioural and stakeholder questions

Describe a time when your data analysis contradicted a stakeholder's gut feeling. How did you handle it?

Approach
  1. Pick a story where you drove the decision, not one where you observed it.
  2. Close with what you would do differently, concretely.
  3. State the situation in two sentences and spend the rest on your reasoning.
Follow-up
  • How did you know the outcome was caused by your change?
  • What would you do differently if you ran that project again?

How do you handle categorical variables with high cardinality in a mac…

medium
behavioural and stakeholder questions

How do you handle categorical variables with high cardinality in a machine learning pipeline?

Approach
  1. Close with what you would do differently, concretely.
  2. State the situation in two sentences and spend the rest on your reasoning.
  3. Name the disagreement or constraint, and how you resolved it with evidence.
Follow-up
  • What would you do differently if you ran that project again?
  • How did you know the outcome was caused by your change?

Turn a one-line fraud-number request into a scoped brief

easy
scopingmetric definitiondenominators

A stakeholder messages: what is our fraud rate, and is it going up? You have fct_payment_authorization, fct_card_dispute and dim_customer. At least four defensible answers exist: count-weighted or value-weighted, attributed to the transaction month or to the dispute filing month, and gross or net of recoveries and successful representments. You get one reply before someone else produces an uncaveated number. Write that reply: the clarifying questions you ask, the single default you will produce if nobody answers, and what the default excludes.

Approach
  1. Establish the decision behind the question first, because a risk-rule change, a board number and a merchant contract negotiation need different denominators, and asking which one is not stalling.
  2. Offer a short menu rather than an open question: a stakeholder can choose between two named options but cannot specify a denominator from scratch.
  3. Commit to a default so the reply is useful even if nobody answers, for example net fraud loss in basis points of settled volume, attributed to the requested_at month, matured months only.
  4. State the exclusions in the same breath as the default: non-fraud dispute categories, transaction months with less than 120 days of maturity, and first-party abuse that arrives coded as consumer_dispute.
  5. Give a delivery time for the default and a longer one for the fuller cut, so the choice between them carries a visible cost.
Follow-up
  • They come back wanting it by merchant for a contract negotiation. What changes in the definition and in the maturity rule?
  • How would you separate first-party abuse from third-party fraud in this data, and what would you refuse to conclude from the split?
  • 01

    Describe a time when your data analysis contradicted a stakeholder's gut feeling. How did you handle it?

  • 02

    How do you handle categorical variables with high cardinality in a machine learning pipeline?

  • 03

    A stakeholder messages: what is our fraud rate, and is it going up? You have fct_payment_authorization, fct_card_dispute and dim_customer. At least four defensible answers exist: count-weighted or value-weighted, attributed to the transaction month or to the dispute filing month, and gross or net of recoveries and successful representments. You get one reply before someone else produces an uncaveated number. Write that reply: the clarifying questions you ask, the single default you will produce if nobody answers, and what the default excludes.

PracHub interview preparation framework ↗
Is this an official Banco Santander interview guide?

No. It is PracHub's own research and practice material for the Data Scientist role at Banco Santander. Rounds and questions reflect what candidates have reported, not a process Banco Santander has published, and they change over time. Confirm the current format and scope with your recruiter.

PracHub interview research ↗
How difficult is the technical interview for this role?

The technical round is known to be very detailed and rigorous. However, the difficulty is balanced by a highly supportive interview environment. Interviewers are looking for depth of knowledge but are also willing to guide you if you clearly communicate your thought process.

PracHub interview research ↗
How much time should I spend preparing?

Most successful candidates spend 2 to 4 weeks preparing. You should split your time evenly between reviewing ML fundamentals, practicing SQL/Python coding problems, and preparing behavioral narratives that highlight your business impact.

PracHub interview research ↗
What differentiates a strong candidate from an average one at Banco Santander?

A strong candidate doesn't just know the math; they know the business. The ability to explain *why* a model is useful to the bank, and how to explain its inner workings to a compliance officer, is a massive differentiator.

PracHub interview research ↗
Will I need to know specific financial regulations?

While you are not expected to be a legal expert, having a basic understanding of model explainability and data privacy (like GDPR) is highly beneficial. You should be prepared to discuss how you ensure your models are fair and interpretable.

PracHub interview research ↗
Sources & methodology 3 sources ↗

No official company page is cited. Rounds and questions come from candidate reports and PracHub editorial material; each source shows the date it was read.